It's easier to "sell" slalom, waves & freestyle to event sponsors (better for spectators). Formula is not conducive for spectators (off shore too far).
However, much of the world doesn't have enough wind for slalom, at least not consistently, so Formula makes sense for most of us.
As SteveC said, sailors go where the money is. Plus, sponsors go where the spectators are, and the spectators go where they can actually see the event.