Because it's not really a Sonic game. This is the same move they did with Sonic's Ultimate Genesis Collection where, again, purely for marketing reasons, they slapped Sonic as the poster boy even though he had a minimal inclusion as opposed to the other Genesis classics included on the disc (this mostly holds true in America. Outside of the U.S., the game was billed as Sega's Ultimate Mega Drive Collection with no nasty Sonic render on the front.)
Sonic's priority on the box is purely to sell off the brand's name. I don't exactly approve of the practice and would prefer the game's title to just be Sega All-Star Racing (this was actually its concept name,) but at the end of the day, Sega's a company and wants money. So, to get the game more attention, they slap on their best-known franchise on the title to increase its selling potential.
In other cases, when we DO get main series titles, complaining this early when all we have are a CGI and a handful of gameplay photos is kind of stupid. It happened with Unleashed, but that turned out to be pretty good... moreso on the PS3 and 360. With Black Knight, the idea was novel but it was really poorly executed and a good chunk of the game wound up being just collecting items to try and get more stuff for weapons Sonic can't even really use.