My reasons for not wanting a character? I'll usually adjust to a character after a while (Rosalina comes to mind), but most of the time, I think mine is just rivaling characters.
For instance:
Bowser Junior: I don't want him because I feel Toad, a character important to the series and has been around since the original Super Mario Bros., would be a better choice for a character who's "uniqueness" is completely based off of a weapon he used in one game. Yet, strangely, this does not affect Krystal for me, maybe because there's really no other Star Fox newcomers popular enough to be considered, and I drastically want variety in the series.
Cranky Kong: I feel that K. Rool is far more deserving, and should get in before Cranky. In fact, even Dixie should get in before Cranky. Eventually, if his playable roles continue, I can see him getting in after Dixie Kong and K. Rool, but otherwise, not really.
Pokemon Newcomer/ Pichu: I believe bringing back Pikachu, Jigglypuff, Red (with Squirtle, Ivysaur and Charizard) and Lucario, alongside Mewtwo, fills Pokemon perfectly.