Although there are a multitude of scare cards that can hit on the turn, we need to raise here. It is very likely that we still have the best hand, and there are a lot of hands that we want to fold behind us. The pot is very big, and although many hands would be correct to call two cold behind us... lets give them a chance to make a mistake and fold. It will be easy to get away from the hand for one big bet if many of the worse scare cards hit. So lets get rid of some overcards and maybe some gutshots/pairs and see what the turn brings.