By the time you reach the river, having bet both the flop and the turn, and having been called, Villain will hold a high concentration of made hands above a certain strength and no air apart from maybe missed draws. When you bet again on the turn, Villain is again supposed to fold the weakest his weakest hands, leading to a further contraction in his range.
This makes his range narrower and stronger than it was previously.
When you bet the flop, you force your opponent to fold some hands.
