1. Because slowplaying the nuts is what fish do. Also fish expect you to slowplay the nuts so they never give you credit for betting the nuts.
2. If he has a flush he's not folding and if he has a flush draw he's not folding and if he has an Ace he's not folding and if he has a Q he's not folding so what's the point of not betting again?
3. The Q is a scary card on this board and he might bluff it to get us off Ax.
4. If he has a draw like KsJx and the river bricks there's no chance he's putting more money in.
5. Makes river shove a lot easier if we bet the turn, he's forced to call off river with a ton of holdings getting good odds