I believe it's mostly the lower Materia population. I think Crystal rank can sometimes pop, but the lower ranks are pretty much dead after a while.

I know someone who couldn't play on Materia last season after a bit because they didn't climb high enough in time. That's why they wanted to climb early this time so they could play on it more. I think there's a Materia PVP discord where people can sometimes plan to play a few games. I'm not part of that but you can maybe ask around in the Wolves Den.