The pools are proxying their hashrate to the biggest players (Ant, Foundry) due to FPPS.
For the smaller pools, paying out FPPS hashprice during a dry spell requires a large bankroll. Hard to get that when you're just earning a 2% pool fee.
I also suspect fee variation due to ordinals/brc-20 etc - where big players get an outsize % of high fee blocks - may play a role.
The current incentives all point to majority hashrate at either Ant or Foundry. That is scary.