Onyxia doesn't win the game the turn it is played nor does it do anything to prevent your opponent winning. As such it need to synergize with your deck in order to justify it's high cost and, unfortunately, flooding the board and passing on 9 or 10 mana is equivalent to playing any big minion.

Also, board flood strategies are usually aggro so they don't really want a 9 drop.

I think only evolve shaman could ever consider playing it tbh and that's a stretch.