It doesn't make sense to include more F-Zero characters at this point. There's no more call to represent F-Zero.
Heck, I'm almost surprised that Cap is still in the game. I actually laughed to myself the first time I scrolled over Cap on the character select screen and thought about how much of a miracle it is that Nintendo still bothers to include him.
All that said, while I'm not the biggest F-Zero fan, I'm glad that Captain Falcon is still playable, as if he's being grandfathered into each successive iteration of Smash. I feel the same about Ness, only difference being that his inclusion means a little bit more because I'm a Mother fanboy.