What you just posted was very results oriented. You didn't know that villain was holding 99 until after the hand played out. Assume that villain's hand is hidden. What could he have that he might call a 3/4 pot bet on the turn with?
* He may have paired his ace: AK,AJ,AT
* He may have two clubs: tons of combos
* Some really bad villains might call with KJ or KQ/JQ-type hands
* He may have two pair or smaller sets: AQ,A8,A3,QQ,33,88 (he would probably raise your turn bet here then and you can then get stacks in)
By betting the turn, you maximize value from the hand if he is holding one of those hands, and can probably then get stacks in on the river easily if he is holding top pair. You also avoid giving him a free draw if he's holding two spades or KJ. If he's not holding one of the above hands, then you'd be hard pressed to get value out of any sort of river bet... like you said, most opponents would probably fold 99 to any sort of river bet on that board.