Sega in 2006/2007 after nearly killing the Sonic franchise with a god awful game: A game with Sonic as the only playable character wouldn't work. The public wouldn't like that.
Sega: Releases Sonic Unleashed, a solo Sonic game in 2008*
*Sonic Unleasehed becomes one of the highest rated Sonic games in a long time and gets relatively good reviews especially from the fans. They think it's a major step in the right direction*
Sega: Releases Sonic colors, a solo Sonic game in 2010
*Sonic Colors gets amazingly good reviews. The fans love it and demand more games like it*
Sega: Releases Sonic Generations in 2011, a solo Sonic game, to celebrate Sonic's 20th anniversary
*The public agrees that it's the best Sonic game in years. Sonic is finally good again and we can love him once more. They welcome Sonic back to the throne of gaming with open arms and can't wait for future releases.*
Sega: Releases Sonic Boom in 2014. A buggy, slow game where you're forced to play as other characters besides Sonic. The game's frame rate is so bad it renders areas unplayable and the music is largely unforgettable. It's the lowest selling Sonic game of all time and pretty much kills the Sonic franchise.
Sega afterwards: Well we don't know what to do. We tried to put too much into that game, but a solo Sonic game just wouldn't work.
Public: Just make another game like Sonic Generations or Sonic colors again!
Sega: Here's our latest game idea, A SEQUEL TO SONIC BOOM! It'll be out in 2016!
amirite?