I think it largely has to do with Sonic's status in his home country, his legacy with Smash, and Sega attempts to diversify themselves beyond the franchise.
It's worth remembering that, in Japan, Sonic just isn't that big. He's still a thing, but he was only significantly relevant to Japanese audiences during the Mega Drive (which came in third place behind Super Famicom and PC Engine) and the Dreamcast (which burned relatively light but fizzled out fast). By far the Sega console with the strongest legacy in Japan is the Saturn, which was a legitimate contender with the PS1 throughout most of its life in Japan thanks to not having a Bernie Stolar attempting to shut down overtly Japanese or 2-D games, which was obviously devoid of any major Sonic titles. As such, Sonic doesn't really have the status of Mario's rival or the face of Sega like in the West, which has hampered the legacy and popularity of the franchise in the land of the Rising Sun.
I think how long he's been in Smash is kinda to his detriment. As the only third party to appear in all four Smash games that include third parties, I think people take Sonic's inclusion for granted more than they do other characters. I think this phenomenon is occurring a lot of the third parties who have been in Smash for a while now, like Mega Man, PAC-Man, and Ryu, but Sonic seems to be hit the hardest by far. People kinda forget how much of a big deal it was to get Sonic in Smash and appearing alongside his former rivals. As such, there are generally less eyes on Sonic's representation in Smash and what another character could bring relative to chasing new faces with no representation in Smash.
It's also worth noting that Sega seems to be divorcing themselves from their image as "those guys who make Sonic games". He's still their mascot and gets a lot of attention, but they're clearly trying to spread the love instead of putting all eggs into the Sonic basket like they did throughout the late 2000s to mid 2010s. It's clear that a lot of manpower has been put in to better associate Sega with its other big properties, like Yakuza, Persona, and Puyo Puyo. It also seems like Sega is experimenting with taking dominant series and bringing them back to spotlight given Shenmue 3 and the revitalization of Sakura Wars. This may impact negotiations between Sega and Nintendo when it comes to Smash, as Sega strives to get more franchises in rather than expanding what's already there.
That being said, I would be so down for another Sonic character. Obviously, Eggman would be great, but I honestly think people are sleeping on Tails. I mean, c'mon, Mario and Luigi vs. Sonic and Tails? That'd be "you can do Cloud vs. Sephiroth in Smash" tier marketing. I also continue to be shocked as to why Shadow was never an echo for Sonic given the ease of implementation.