[Sephus]

because you let them see the turn without tying them to the pot. you don't want them to hit on the turn or want to fold it if a scare card (for them) comes. fish aren't planning ahead like "oh, if i call $30 i'm going to want to call anything on the turn because the pot will be big so i better fold if i dont want to go allin" they're thinking "i have something, i need to call."

and notice that MP called the flop, and folded the turn? classic "i flopped a pair in a raised pot, oops it's not good i have to fold the turn" move. another reason to bet bigger on the flop. until you bet the turn they're always putting you on AK.