I'm sure money is a big part of why MLB hasn't done something about it.
That said, there's no interference in other sports because baseball is unique to fan interference, isn't it? For several reasons, IMO. You don't have players trying to hit, kick, etc. the ball out of the field of play in other sports, like batters hitting home runs. In other sports, I believe that once the ball is off the field of play, or out of bounds, it's a dead ball. Not the case with baseball.