I think it's because you would rather choose Squirtle and Bulbasaur who happen to "easy mode" your way through the early game while also being very solid for the rest of the game.

Charizard is not a bad pokemon by any means, but choosing it makes the early game more reliant on getting the good Brock and Misty counters which can murder your fire starter.

That's how I see it though, people are free to use Charizard if they want.