Another thing is that what you find in mainstream and what you find in the self-published (of which webcomics are part of that) are often different. When vampires were all over the mainstream, they weren't everywhere in self-published works because people could find it in mainstream stuff and so didn't need to create the content they wanted to see. Then when vamps went out of favour in mainstream, they grew in self-pub. The markets compensate for each other.
So when you see a lot of shounen or action being catered for in the mainstream, you won't find it so much in online comics. Similarly - romance comics aren't a big thing in the big comics publishing houses, so it's huge online for people who want that type of content. It's just basic market compensation.