Mainstream products are known, have an established consistent product that people can rely on, and they're just all around safe.
People that pick up a super hero comic from Marvel or DC know exactly what they're getting it to when they open that comic. There's a main character that they already like, a story plot that has been around for decades, and there's a satisfying conclusion most often of the hero saving the day.
For OC comics the readers/publishers take a risk because everything about it is unknown. Who the character(s) are, what the story is about, if it'll last till the conclusion of the series.
Webcomics are easier for original content to get noticed and gain a following. With webcomics it's easier to take the risk and read something new because they're free, quick to access/read, and OC makes up the majority of webcomics out there. Webcomics are an equal playing field of experimental comics that cover a wide range of all story topics.
In order to get noticed in webcomics it just takes time, patience, persistence, and lots of learning effective promotion techniques. Promoting, Promoting, Promoting!