Because his dialogue is "Lol so random" ...seriously his fans are cut from the same cloth as Invader Zim fans.
You can say you don't like him without insulting or grouping his fanbase.
Normally I wouldn't even touch something like this because it's not really that offensive, but it's still pretty dickish and I hate the mentality behind it. Do you know what the Sonic fanbase looks like to most people?
Anyway, while I do like Fawful, I don't think he would really fit in well with the rest of the cast. You know who I would like to see though? Shy Guy. Shy Guys raced in the Mario Kart Brawl stage, and you could play as one in MKDS if you were only playing with one card. They really should do it as long as they're adding generic enemies.