Sonic's just such an easy target to dunk on. There's so much material you can chose from.
The problem, really, is that people continue ****ting on Sonic even when the franchise does things right (also that Sonic Team keep listening to vocal minorities).
Generally this is because the fanbase is so huge (and so fractured) that Sega has left Sonic Team with no "right answer" for pleasing the fanbase, even the Sonic Golden Boy, Sonic Mania, had detractors.
Say what you will about Nintendo, but Mario has had a
very consistent tone and playstyle for pretty much 3 decades at this point, if Super Mario does something wrong all they need to do is go back to basics and fans will be happy (this was the core idea behind New Super Mario Bros. ...you know before New Super Mario Bros. became same-y). Even when the playstyle is different with Mario (i.e. the Spinoffs), the tone is kept in tact.
Sonic's tone and playstyle has been all over the place from, like,
1997. By that time we not only had the three (and a half) original Games (Sonic 1, 2, 3 and & Knuckles), but Sonic CD (which has a different core design to the others while still using the same engine), Sonic 3D, Knuckles Chaotix, Sonic R, the Master System/Game Gear titles, Sonic Labyrinth, the two Tails Games. Not to mention two entirely tonally different TV Series (SATAM and Adventures of Sonic the Hedgehog), along with two different Comic Book series (Archie, adapting SATAM, and Sonic the Comic) along with the anime OVA, which is, again. tonally different from any of the others.
And this was just within
6 years of Sonic's inception. Five different continuities (the Games/Japanese canon, the OVA, AoSTH, Archie/SATAM and Sonic the Comic), all with their own fans.
After that you got Sonic Underground (another tonally different tv series), as well as Sonic Adventure, Sonic Adventure 2 and Sonic Heroes, all of which use slightly different playstyles with Sonic. And then the Boost Formula comes along. And then there's
Sonic Boom. And now the
Sonic Movie.
Even the much-maligned Shadow the Hedgehog and Sonic '06 have their fans.
Where do Sonic Team go with Sonic? Do they go back to Classic Gameplay? That'll just frustrate the people who like the 3D games. Do they go back to the Adventure style? That'll piss off people too. The Boost Gameplay is a huge tax on resources, Sonic Lost World had problems.
TL;DR is that the reason there is so material to "dunk on" Sonic with is because Sonic, as a franchise, is some kind of Eldritch Monstrosity that no one fan can agree on.