Good question. It's mainly just the way we've got things coded right now. We need a fix schedule at which to run our reward disbursement algorithm, and block confirmation made the most sense as that is when the pool receives income, thusly maximizing the total possible payout. Additionally, when we first switched over to ESMPPS we had 0 buffer (obviously) so for the first round (at least) we had to wait for the block to get confirmed.

Right now, if a block is invalidated, the shares from that round are included on the next rounds payouts.