Sonic Worlds was one of the early actually-pretty-accurate frameworks/engines so a bunch of people started using that and making their own edits of it, and some wanted to make more accurate ones so here we aaare, it was difficult to find anything properly Sonic-like to use before Damizean and the gang started working on Sonic Worlds like 15 years ago. Fusion definitely isn't as popular anymore I don't think, noone really seems to speak of it outside Sonic and FNAF fandoms with Godot and GameMaker (and Unity) becoming more popular. But uh yeah, it's just been around for so long many wanted to and still wanna use it.
Just wanna add that Fusion is also the most easiest to use out the other programs that I have tried.
God knows how many years ago when I wanted to try to do something game dev related with 0 knowledge I tried RPG Maker, then I tried GM 4.0-8.0.
But the UI and how to make anything whatsoever was too painfully confusing for me.
Later saw Unity, Unreal and GM Studio... Nope wanted none of that either.
But Fusion somehow instantly clicked with me, like I could figure almost everything out on my own, mostly simple and can just test any part of the game with 1 click of a button no extra hassle and virtually do anything I want outside of some super complex stuff that needs figuring out.
Now is it perfect? Ofc not none of the game making programs are.
But generally speaking Sonic was always Fusion first, GM4.0-8.0 second "Now being GM Studio"
And rest is just whatever people wanna make their "3D" engines on.
Aaand I'll add to that one that I too am too dumb for any real coding lmao, I think I first used Klik & Play at school during some of the PC classes and yeah, then got to using Multimedia Fusion 1.5 and stuff onwards next. For me too it kinda just clicked and continues to be comfy, yeeeah it's limited here and there but I'm managing pretty well.
I did watch an old bud once use Construct 2 aswell and yeah, that looked simple in the same sorta way as Fusion is, but I've never heard of a proper Sonic fangame (as in with physics and so on) made with it.