SFIV was superficial enough of a return to "form" that it looked amazing from a distance. All SF's are different beasts, but basically the first SFIII entry came out I think sometime around SF Alpha 3 and it was a game different enough from Alpha/SF2 that it alienated people on a casual level. Mostly new characters (this is important because Alpha series had new characters but still a bunch of familiar styles/char), choose a super/length of meter, EX move in 2nd Impact, parry, etc. Coincidentally around this time fighting games started to lose popularity according to Yoshinori Ono so people didn't focus on them as much in the mainstream.
Plus SFIV had more casual friendly things, like character reactions to Ultras on their faces, some cartoonish Ultras (Akuma Ultra 2?), Ultra system on top of Super meter where you will by default have access to an Ultra art if you are hit enough (base damage for half-filled ultra is 100%, full ultra is 150%). Focus attack, I don't know what to think tbh. Casual players probably either abuse or don't use it even if it only absorb on hit. SFIV does have some kinda hardcore stuff like one frame links though
To say nothing of whether all this was good or bad. It was just a strategic overture to corralling a lost audience
butIdon'treallylikeSFIVallthatmuch