That doesn't mean they couldn't have had Sonic AND those franchises. And what of Jet Set Radio? What of Golden Axe? Of Phantasy Star? So much forgotten behind. And even then, that doesn't excuse Namco and Capcom missing so many characters.
Sonic would've overshadowed everyone and would stick out like a sore thumb with the playable cast where everyone is human or humanoid.
Golden Axe actually gets a reference in the form of one of Bruno's attack. Meanwhile, Ulala's attack references like three or four games.
It may not be the fanservice you were wanting, but there is plenty of it. Sega just decided to represent themselves with their lesser known series, Virtua Fighter, Shining Force, Valkyria Chronicles, Resonance of Fate, and particularly Sakura Wars which got two full teams representing that series - a feat no other series performs in the game.