I know the Senshi's minds were wiped or whatever. They don't remember being Senshi or meeting each other. Was it ever explained just what caused this?

Also, were they thrown back in a time to re-live that time period as normal girls? Or did they just continue on from the battle against Materia with really creative, selective amnesia?

Whenever the silver crystal is used to its fullest, it lets the bearer make a wish. Usagi wished that they went back to being normal girls, and so they never remembered being senshi.

I think it's the super-selective creative amnesia, since the silver crystal doesn't possess time-traveling capabilities.

Wait...
Queen Serenity wished for all of the people of the past to be reincarnated when she destroyed the Dark Kingdom.
Usagi wished for them to have normal lives when she destroyed Metallia.
She then wished for Luna to become human for a short while when she killed Kaguya.
Any wishes I'm missing?

She also wished to protect her friends at the end of the R movie just before she died. But I think that was it. xD

As Makoto still attends Usagi's school, we can assume they just forgot

My question is wouldn't Usagi remember meeting them though? She didn't think Rei, Ami, and Mako were senshi.

I think not really, if I remember correctly, in the first episode of R, she seemed to be acquainted with Ami and recognised Makoto because they all go to the same school but they didn't hang out or anything. She didn't know anything about it until she had her memories returned. She probably met Makoto and Ami again in between the two series.

Well, I feel like Usagi would at least remember meeting Mako the day she transferred to her school. :/

She probably had false memories, or just a generalization without specifics. She knows them, but doesn't remember knowing them, you know?

Like you know someone's name and stuff but you don't remember the specifics of how you met, and because they're not super important to you, you don't stress about not remembering how you met or really ever have an occasion to think about it.

But why would Usagi's wish eliminate their memories of their friendship? If the amnesia is selective enough to let the girls remember they were actually doing things for the past few months (instead of it all being a big blank), then it could have left their non-Senshi friendship moments intact as well fairly easily, I would think.

Tuxedo Mark wrote:

But why would Usagi's wish eliminate their memories of their friendship? If the amnesia is selective enough to let the girls remember they were actually doing things for the past few months (instead of it all being a big blank), then it could have left their non-Senshi friendship moments intact as well fairly easily, I would think.

I believe she sacrificed their friendship in order to let them live normal lives or else it was just a side effect and they were destined to become friends again regardless. Sort of like how Mamoru and Usagi are constantly thrust apart only for destiny to bring them together.

Tuxedo Mark wrote:

But why would Usagi's wish eliminate their memories
of their friendship? If the amnesia is selective enough to let the
girls remember they were actually doing things for the past few months
(instead of it all being a big blank), then it could have left their
non-Senshi friendship moments intact as well fairly easily, I would
think.

But they only really became friends *because* they were senshi, it is what brought them together in the beginning. Without that, they would have never had that initial reason to get to know each other, right?

Mostly yes, but Usagi and Mako bonded before we learn Mako is a senshi.