Well, when you choose to play a character that you are aware is lackluster compared to the rest of the cast, you willingly accept the immediate handicap having to play/work a lot harder.
And you know what? It's worth that struggle,
And by struggle, there's barely one to begin with. While it's known Sheik and Diddy are top tier, the game has been out for less than a year. Diddy is already becoming easier to matchup against since December, and Charizard is a far, far better character than he was in Brawl.
But in general, I'll dignify a loss using the character that represents me over a win using a character and becoming a dime-a-dozen player who resorts to the winning character. Why struggle to learn someone who doesn't fit my style when I enjoy the game far more using my favorites?
I'm gonna guess you don't actually go to tournaments.
You're allowed to play whoever you want. So is everyone else. Their reasons for doing so are as equally valid as your reasons for not doing so.
That is an incredibly ignorant guess and a myopic representation of the tournament scene.
I was at Olympus. I saw one Sheik and no Diddy players. If there were any, they didn't place. The most we had were several Captain Falcon players, and the winner was a Pikachu.