Quite a few people have asked me why certain characters are banned from tournaments like the EVO Fighting Game Championships.
Gill is the boss character from Street Fighter 3 Third Strike, and he's banned every year from competition. He is considered by almost everyone to be a broken character, as evidenced by this video.
A lot of thought typically goes in to which characters will be banned, if any, from major tournaments. At this year's EVO competition, out of the six games being played: Street Fighter 4, Super Street Fighter 2 Turbo HD Remix, Marvel vs. Capcom 2, Soul Calibur IV and Guilty Gear Accent Core, it's not unusual to see a couple banned characters from some of the games, except for the cases of Street Fighter IV and Marvel vs. Capcom 2 where everyone is selectable.
While some players will always complain certain characters are too good or too weak, by and large characters typically aren't banned from EVO unless they are highly unfair.
And if players were allowed to select these characters, you'd likely end up seeing most match ups being Gill vs. Gill or Akuma vs. Akuma for the majority of the tournament.