UFC and Pride DID try working together. This was right around the time that UFC sent Liddell over for the tourney. On one of the UFC PPV's, they even announced that Sakuraba was going to fight in UFC. That obviously crashed and burned, and nothing else was ever mentioned by either Pride or UFC.
Pride still suffers from low PPV buy rates here (I've heard as low as 10,000). They have a 'best of' type show on a sports channel, but they never really do much as far as PR. They need to be jumping on the bandwagon, since MMA (UFC mainly) is bringing in a ton of new fans (in large part due to a UFC reality show). Pride was supposed to run a card in Las Vegas...never happened.
K1 has never really caught on big, despite having some live matches on a sports channel here. Same thing as Pride...lack of PR. They've ran some strong cards in Las Vegas, too.
Since the state of California started allowing MMA competitions recently, an independent group drew a huge gate for a show. The last UFC card and the next show is in California. It's probably the huge untapped market for MMA as far as the U.S. I expect California to become the next Las Vegas as far as MMA.