afaict three reasons:

1. OCEAN uses a payout system that's similar to PPLNS which has relatively high payout variance relative to PPS/FPPS (which small pools can't provide because it's very risky/expensive), and big miners want less variance all things being equal
2. To mine on OCEAN, you also have to run a bitcoin node so that you can build templates (which is a good thing), but more friction = less customers
3. The pool is relatively young and network effects are in play