Honestly I think it comes down to Geno being perceived as a cool character, and SMRPG being held in high regard by a devoted subset of fans. That's how it kicked off, at least. But it was enough for Sakurai to take note. Now the "cool factor" has maintained, but I'd guess what has really guided Geno for a long time is the belief that he's likely, built partially on the backs of developments like the costume, Sakurai's statement, and Square's involvement. Before those elements, support was definitely waning. Still, I'd reckon a good portion of his fanbase hasn't actually played his game.
People flock to characters they believe stand a good chance. For example, the support for Doom Slayer between now and two years ago is night and day. Even though nothing about the character themselves has changed in that time-span. Nor, really, has the Nintendo support (not that that really matters). People say his series is seminal and the character "iconic". But that's as true now as it has been the entirety of Smash's existence. Then you get a Smash acknowledgement from Bethesda and some new releases on the Switch, perception changes, and popularity suddenly follows.
It's a self-fulfilling prophecy. People deem a character likely, enough people buy in, that character's popularity raises to the point it becomes true. Or at least, truer than it was. Geno had an appealing design with attractive moveset potential, so even people unfamiliar with his game agreed he would be nice to see. Soon the movement becomes larger than the fans of the game it began with, and new people are retroactively playing the game and becoming fans themselves. I've noticed a similar phenomenon within the Isaac thread, where people say they're trying Golden Sun out because of how the fanbase raves about it, then many become fans in their own right.
Despite all that, his situation is an anomaly. Even for diehard fanbase characters. At least characters like K. Rool, Isaac, Ridley... how they accrued popularity is more readily apparent.