Because people want to look at or look like aesthetic semi attainable physiques not bloated unaesthetic 4 grams a week bodies.
This.
Lots of guys lift for pussy. You look at an IFBB pro, you're looking at big giant physique that takes a lot of time, training, and drugs, and isn't what women want.
These aesthetic guys, especially the ones that got on youtube early, hit the trifecta as far as subscribers - Guys looking to get buff, schmoes looking for something else, and some women. They also have better out reach and pump out better content visually than most bodybuilders.
Jay Cutler's web presence sucked till he got a real team behind him. Very few want to watch straight ahead type bodybuilding stuff.
Edit - forgot I posted in this already

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