Ok, so like, here's my theory (and it encompasses the difference in schooling in the US vs Japan and elsewhere because I legit didn't know this factoid until I stumbled across it in research.):

Because Usagi chose to continue her schooling to physically go to High School instead of just getting a job after she finished Jr. High/Middle school and wait until she was old enough to marry Mamoru is what caused Crystal Tokyo.

My reasoning?   Usagi doesn't like school. She doesn't enjoy it, it looks like she's bored most of the time. She also never seems to have a specific career she wants to obtain and a lot of times, School just seems like either a hangout spot or something she has to do because, well, she must. Nothing really is her choice in that.

If she never met the others and only met Mamoru, she probably would've just gotten a part-time job and waited until she could fulfill her dream of being a homemaker and a good wife.  Which, really was her original dream to begin with. 

My guess, ultimately, is that because of the -other- senshi's influence (and possibly because she did travel to the future and saw what that future/maybe alternative future?,) that she continued on to High school with them in order to graduate with them too.  (and also because she may have figured she needed more or was convinced perhaps more schooling was needed if she wanted that future to come to pass.).

I say this, because from what I've read up or at least crossed check that, as of this moment/currently, compulsory schooling for the average Japanese student goes from K-9th grade (Basically Elementary school to Jr. High/Secondary/Middle school.)  and afterwards you have the option of going onward to high school (but you need to take exams, a lot like some colleges.) or you could just not go and get a job. Or you could just work part time and then do night school and get it that way at your own pace.

I feel like that due to the Black Moon Arc occurring, that it was a kind of self-fulfilling prophecy in a way because it planted the seed/idea that Usagi may need/want to continue if she wanted the future she saw to happen.

I particularly feel it's a thing she would've done in the 90s Anime and in SMC/Manga but not in PGSM.

90s Anime Usagi definitely was the least interested in school but she's the most easily influenced by her friends and Mamoru in that version. She took a lot of things to heart, at least in my opinion, so if the girls and Mamoru were collectively together on the idea that maybe she should attend High school, for the future or something, she probably would've gone along with it.  

Manga/SMC Usagi isn't as disinterested in schooling as it just seems she just was bored. Academically, she could do a lot better very easily and doesn't actually seem like she struggled as much as her 90s counterpart to understand concept of things too often. She's also not entirely as easily influenced but it's there a bit.  I think continuing on to high school (maybe a year or two in college of some kind or maybe just taking classes at night? ), would've been more her idea than anyone else's.  She seems the type to determine what she'd want.

PGSM Usagi- I'm pretty sure the character is in middle school and she doesn't continue on to high school or college, actually.  Instead, the next time we see her it's a few years later(??) and she's planning her wedding. Which also means she got to live the dream she wanted in being like her own mother. (and the only version of Usagi aside from Parallel Moon, to achieve this dream.)

Speaking of Parallel Moon, this may be why Usagi's pretty young to have two children already -Chibiusa and Kousagi, because she opted to just settle down instead. 

Spoiler:

(I also theorize that this particular timeline is one where Chibiusa never came back in time either. )


TL;DR: 

Because Usagi decided to go to High school, possibly because she was influenced by her friends and lover to do so, it brought about a new path and lead to the formation of Crystal Tokyo and her role as Queen to Mamoru's King.  

But, as they say- That's just a theory.  A Sailor Moon Theory.

:0000 I know it was a bit off topic from the main theory you're presenting, but I really like your thoughts on Parallel Sailor Moon!!! It totally sounds like it makes sense!!

As for the main theory, I do particularly agree with your breakdown as to how Usagi comes to the decision to go (oor not) to high school in each version.

I'm sure there are so many factors that lead to Crystal Tokyo coming to be. Why not add this one to the list ^_~. Thanks for sharing!

Weirdly, or not weirdly enough, Parallel Moon actually helped me come up with this theory in a way.  I kept thinking how (aside from speculated/other factors) in that version, Usagi never became Queen, there's no Crystal Tokyo, everyone's just living more or less ordinary lives, right?

I kept wondering about it and remembered that during the Super arc (Infinity Arc) that the girls (Manga wise) were in their last year of Jr. High School/Middle school.  Which, for them, is what the US would be 9th grade- aka, first year of high school for those of us in the US (and other places that have the same schooling system), making them all Fifteen.  

All the Inners turned, more or less, Fifteen that year, which ends their compulsory schooling. After that year they legit don't have to do any further schooling if they don't want to.  (Rei doesn't have to do any exams anyway, because her school is an elevator one- aka a lot like US schools in which she just gets promoted to the next grade level anyway.)

SO-

I kept pondering what the differences were in Parallel Moon and in the Canon continuity were.  I had to first get rid of the hard-pressed feeling of keeping with the canon timeline and realize it's Parallel Moon- it's an AU that could take place a little earlier than what's normal. It also means a lot of the canon beats (Like Usagi getting married at Twenty-one to give birth to Chibiusa later and turn twenty-two.) might also not apply at all. 

Like, for instance,  Usagi and the Inners being born a decade or so earlier in 1967 instead of in 1978. (estimated/what most people agreed would've been their birth year with Mamoru's being in 1974-1975-ish). 

There's no rule in-story of Parallel Moon that says this could be wrong. We only get one time stamp there and that's the beginning (when this short story came out) of July 1999.  (Which is amusingly the month before Mamoru's birthday and a month after Usagi and Chibiusa's birthday.)

Chibiusa and Hotaru's given age would've been around 14-15yrs old (Mostly likely the latter) because it's stated that both girls are in their Third Year of Jr.High/Middle school. <- This here gives me an idea of when Chibiusa could've been possibly born, which, in turn gives me a rough idea of Usagi's current age in that one. 

And I estimated about 17yrs old would've been when Usagi could've/would've given birth to Chibiusa. 

This makes a lot of sense, if you factor in the aforementioned Compulsory Schooling and the legal age it is for people to get married without (or with) the consent of their parents.  Currently it's 16 for girls and 18 for boys over there (I think. I could be utterly wrong. Laws change over time but at the time of the Manga's in-story years, this would've been correct, I believe.).  Which is kinda roughly the same in some parts of the states depending, of course. (even though overall it's 18 here but I digress.)

Anyway-

My point being is that once I got rid of the notion that I absolute had to abhere to the main canon storyline time stamps, I found it a lot easier to estimate everyone's ages- and I used Naoko Takeuchi's birth year because she would've been that age at the time she wrote Parallel Moon.

I also had to go back and remember that during the SuperS/Dream Act (and it's been repeated before) what Usagi's actual Dream was that had nothing to do with her Sailor Moon/Princess things.   

Her original dream was literally stated in the Dream Arct manga as being a Homemaker/Good wife and for her children to be happy and to have a nice home.   (Because, her mom. She wanted to be like Ikuko and that's pretty cool.)

In Parallel Moon, she literally achieves this goal. 

So, I figured that both schooling and her getting married earlier probably is what changed here between the Canon and the Parallel Moon world. 

Plus, she still gets to meet the Outers anyway since Infinity arc happens while she was still in Jr. High. We can assume that Setsuna, like the other girls, was reborn similarly to Haruka and Michiru.

Spoiler:

Long story short (lol) again:

Usagi choosing to continue her education and waiting until she was older than seventeen is most likely one of the reasons why Crystal Tokyo would've been way more likely to happen to her in the Canon Storyline.  Whereas Parallel Moon, she settled down after schooling, maybe picked up a two-year thing and just lived life, thus Crystal Tokyo doesn't happen.

(other factors notwithstanding.)