This idea that Sonic is "under represented" in fighters didn't come around until the Sephiroth DLC.
It was a sentiment among more casual fans for a long time, and probably started to manifest more passionately after Shadow was passed up as an echo fighter and we got Ken and Richter. But you're right that Sephiroth gave them a second wind, since most of us had assumed they would be adding entirely new series or unrepresented games as DLC anyway.
But yeah, something I see people neglect as well is that Final Fantasy does pretty equal numbers to Sonic on a world scale, and outright tramples it in Japan. It actually edges Sonic out by about 25 mil units sold, if that counts for anything - so if we're talking on the premise of cold hard statistics, it adds up. People like to act as if Sonic is the biggest third party in the game by a country mile. It's up there, but it's a misconception. He's the most ""fitting"" one on paper because of his deep history with Mario and the fact that cartoony platforming mascots are always going to be more easily embraced in Smash, but I too often see people use this as a way to act entitled to another character and suggest it has earned special treatment over other guests.
Like, I agree that it's in pretty high billing to get a new character next game, but you get what I mean. No disrespect to anyone in this thread either, I was just speaking generally about the fanbase and how they treat Sonic's representation. Next game may be the first time I really feel like that energy toward a second Sonic character is warranted / justified.
Nintendo chose Ballot favorites or lucrative promo picks. Sometimes both. A Sonic character would honestly be neither.
I do wonder how well these guys did in the Ballot though. Eggman support didn't kick in til later, so I'm going to consider that a nonfactor. But Tails has been requested for about as long as Sonic has, and Shadow / Knuckles definitely had some support as far back as Brawl. So I wouldn't be surprised if they performed decently well, but I also wouldn't be surprised if they individually kind of splintered each other's votes.