Because it was inevitable.
Pornotube, and in a similar vein, Youporn are free adult community-driven websites supported by relentless public uploads. The principle is similar to Youtube, which is of course where they parody the name from. However, you don't tend to get much hot lesbian action on Youtube.
Unless I'm looking in the wrong place.
Due to the...eclectic tastes of pretty much everyone on the internet, you can pretty much find any kink that floats your boat. Whips. deepthroating, midgets...I think there was even some clown porn at one point...
/me shudders
The main kicker about the website is that it supplies high-quality streamed video (often long-running ones as well) completely free, due to a hearty dose of advertising bars, usually for other adult services. In theory this is fine, but having a gay porn banner flash up just before the vinegar stroke can be a little offputting (unless you're into that kind of thing. We advocate equal porn rights here at E2!).
Pornotube! Bringing free porn to the (sticky) masses!