First thought is way too many hands that beat you. Second thought is that you almost never have a flush with this line (I suppose, being very deep, you might get tricky on the flop, but the flop/turn play makes your line make a baby flush rather unlikely IMO).
I guess he figures if you have a big ace with a
you're going to check back the turn a lot, and he has to figure that on this board with that flop bet you are going to be floating quite often, so his c/r could still be a somewhat wide range. It's also probably unlikely that he would play AA like this, and even a turned straight would probably be discounted since, again, you have a single
and are checking behind quite often.
Pot is laying you a decent but not great price, but the main issue here is that sometimes this is nuts or nothing, but if dynamic indicates that either of you might be about to get out of line, this is the best line for him to take with a flush.