multiple reasons actually:

- pot control, pot control, pot control... <-- one of the most important concepts in NLHE

- i have him on other a set, or a club draw, and thers a small chance he could have QJ for a straight draw as well, or a weaker Ace.
Therefore ur either way ahead or way behind.

- if he has a club draw hes drawing dead, if he has a straight draw i have 2 of his outs covered so hes drawing very thin, if he has a weaker Ace hes also drawing thin. By betting this turn, your allowing him to fold everything that you beat, and raising/calling with everything that beats you. Unless you bet it very weakly, like half pot or less so that the draws have the odds to call u down, but if u do this, then any smart villian will take it as a sign of weakness and therefore check-raise your small turn bet regardless if he has a set or not. If he does this, then you will have a harder time playing the turn, and if you call the turn raise, the river will be bloated, and thus you will have an even harder time playing the river.

- induce a bluff from villian on the river, which i can easily call down on the river if i miss my club (because i controlled the size of the pot by checking behind on the turn).

- by betting this turn, you risk getting check-raised by villian if he has a set. by checking behind, you take that cr away from him, and in the process minimize your losses vs the set (implied odds) while maximizing your profits vs a drawing hand, by allowing him to bluff a missed draw on the river.

- villian flat called a near potsized bet on the flop from the pf aggressor in a raised pot --> this means he either has u drawing dead, or he has a draw and your ahead, with a slight % chance that he has a weaker TP (AQ).

- betting the turn imo = -ev, because you will no longer get any value from a weaker hand, but u instead stand to lose when ur behind.