Because there are a ton of cards that will have him check/calling the river if he has a two pair / set-esque hand. Any 9, any 10, any A, maybe a spade, and also if the board pairs and counterfits his two pair. Nearly half the deck. If the board pairs and he doesnt fill up but still bets, you arent raising - so you still miss out on one bet.
If the river is a complete blank, there's a good chance that he bets the river after you cap anyways.
You save a bet when he fills up. You usually miss out on a bet when he doesnt.
There are far more instances of the latter than the former.
What 2 pair is getting counterfeited here? KJs/QJs, but that only makes up like 5% of his range and only gets counterfeited less than 10% of the time. Besides, if one of those counterfeiting cards does hit the river, I will be happy that I never capped since I'm now way behind to his range.
I also think that if I cap the turn, it will slow down his set/2 pair hands on the river. He may bet, but he won't 3-bet.
With regards to a scare card hitting, that only happens about 20% of the time, so it isn't too much to worry about - especially since I'm only ahead here like 2/3's of the time anyway. When I can pin donw his range so narrowly, I think it is better to wait for the river card, since it will allow me ot play more optimally against him.