The main issue is that they took too long to gain much of a footing (reaching their peak in the 2000s) before other markets came in to snatch up a lot of the comic-reading audience that didn't care for cape comics. Not to mention, when the internet was getting big, those "alternative" comics weren't really posted online much compared to other foreign offerings, there are entire generations of comic-readers who have literally never read a western comic because they started elsewhere when they were a tween and never looked back.