I personally think it was just because thanos had beat the crap out of him at the beginning or the movie and he was too scared to come out now. JK! In reality I found it a little off putting given the fact that if you look a little into hulk's history, its abunduntly clear that the more he gets beat up, the angrier he gets and the angrier he gets the stronger he becomes. But I understand that the movies don't particularly follow the comics to the dot thus we can only wait and watch as to what the MCU comes up with.