Better question:
Why should I (or anybody) help a website that makes money off other people's work?
Let's say I gain superpowers and make a character. Many days of work go into the barely passable character I made. Everything's fine, download link is still available, good shit. Other people upload their weeks worth of creations, hosting from various sites, whether it be OneDrive, Dropbox, or their own. Some of which they pay for with their own money. Then a website comes in to gather all the Mugen content it can find. It uses a service that uses ads (with no viruses) to make money off of the work I (and many other creators) have done.
Why should people help that site?