It's not DC, it's a combination of DC, Cartoon Network and Warner Brothers, all daughter companies to Time Warner. They're companies. They care about profit and intellectual property. Not what a bunch of fans want. YJ had a reasonable run on comics, online episodes and dvd sales, but the toyline totally tanked, and that is the main source of profit. It was an expensive show to make that didn't hit the proper demographic. Quality didn't have anything to do with not renewing it.
There haven't been "enough people" for years, and the companies have said they're not open to fan funding either. As for making your own stuff... that's essentially fan art meets fan fic. There's so much fan fic already, it's really rather arrogant to declare yourself the ultimate fan fic writer and come up with an "official" season 3 fan fic. It would be a kick in the face to all the other fan fic writers, not to mention Greg, Brandon and the other writers on the show.