By betting the turn he can raise you with sets, two pair, bluffs AND flush's. If he does this you have a nice out of control pot with THe nuts, which is exactly what you want. If you don't bet the turn the pot is still nicely in control, if he bets the turn and you checkraise you make your hand brutally obvious and ONLY get paid off by other flushes. You are giving the most hands an opportunity to fold whilst only getting marginally more value out of him by trying to check raise.