The number of MN decreased considerably from ~2200 to ~1550, that might explain the lower reward for staking, I guess a number of people switched from MN to staking because that was more profitable the last 5 weeks.
Maybe a bit of that, but most of the drop occurred in the last couple of days. I'm guessing it's more to do with MNs going offline because they're still coming back after being upgraded to 2.3.0 (or because they still haven't been upgraded).