He's a character inspired by old cartoons from the early half of the 1900s who within the first 3 years of his existence hada looney tunes-esque show made by the people thatmade inspector gadget. The first few mainline games are basically just "ok but what if mario had EVEN MORE PHYSICS" and due to this are really fucking solid and HAD ports on almost everything ( to varying degrees of bugginess till origins came and they deleted all the ports for their new buggiest port)
The spinoffs of varying quality all fill niches that people to this day appreciate despite journalists and youtubers playing up all of them as hot garbage instead of taking time to learn the controls of or dealing with the batshit difficulty of some of the better ones never stops being funny yet depressing to me.
Sonic adventure is a really easy to digest game where you go through the same stages with a bunch of different characters with their own goals and gameplay styles, the ports of the already buggy DX port have ultimately tainted how internet video essay fuckers view the game but like if you go in blind the only issues you're gonna have unless you fail to pick up on blatant clues of what to do is shit like snake room and maybe big's story if you hate fishing minigames.
Sonic adventure 2 took the gameplay of sonic aventure 1 and refined it , making more linear stages with stuff hidden if you go off the beaten path and axing the overworld which despite being a fun feature wouldn't really have as much sense being in 2 as it did 1 since 2 is more a globe-trotting kinda journey while 1 is exploring a very specific city angel island crashed near. Video essayists now bitch about the mech stages and claim the game isn't good because no overworld but it's not stopping people from buying and playing the buggy ports of the game en masse even to this day which kinda says something bout society though I'm not sure what.
The art style. The Adventure redesign was so good that it's still in use today. Only thing that has been changed over the years is how long Sonic's quills are. Adventure era Sonic had really long quills.
Green eyes is really the only true design change aside from BLARMS but yeah the quill length and leg/hand size weirdly vary throughout the years from game to game. The quill thing though is sometimes referred to when professional video game people talk about modern sonic so I get why the quills would be brought up.
Overwhelming sexual attraction towards the characters compels the players to stay in an unhealthy relationship with the games.
I wonder sometimes if those guys even buy or play the games. There's like an entire subgenre of fuckers out there infamous for unironically getting pissed ver green eyes on the blue rodent and being extremely horny for the DiC cartoon/Archie comics exclusive chipmunk GF sonic had.
But yeah Tl;DR Same reason Mario is still popular. People tend to like the jumpy run through obstacles kinds of games. Bing bing wahoo.