I think this is the best reason. Like the latter statement by SuperRad, Falcon is the funnest character in the game. If he's also your best character why would you want to play anyone else? haha Falcon *****.
falco and puff are the most fun, they have the most situations to adapt and play differently to and all their moves have good purposes that can be used in a lot of different ways. They also have very unique playstyles, nobody else controls the stage with lasers or can move in the air that well, and both have really fun shield pressure techniques and we all know shield pressure is the most fun part about this game.
Marth and sheik have to be the most boring if you main them. Obviously if you played puff all your life and you just picked up marth you'll have more fun with marth but if all things are equal then they are pretty boring, just camp till you get a grab then combo traditionally until you can gimp or edgeguard in a way people consider gay.
But then again, if you do something new with marth or sheik it looks tight and you seem more innovative than usual.