Because they're in win-now mode and Moncada would take longer to develop than Benintendi. Plus, he was blocked by Pedroia.
Benintendi is better right now, but it's very easy to see Moncada being the more valuable player in the long run, especially given Benintendi's average-at-best defense at left field, which is either the least important or next to least important (1B) defensive position.