Diaz wants more money, he doesn't feel it right to sign a zuffa contract, which doesn't let you participate in other events. I am sure if he got offered Rashad Evans money, he would gladly sign.
I mean, Rashad gets paid 400,000 and I don't even know why.
Because he's a huge name and has had a pretty exciting rise to the top, beating some of the top LHWs. Why shouldn't he earn a big wage?
Diaz is probably looking for a big money contract because he knows he'll get dominated by various UFC welterweights and so his hype will be short lived. These days wrestlers have sub defense (minus Chael Sonnen) and can simply take guys down and grind them down for 3 or 5 rounds. It's ugly, but it works.
Diaz would get the wrestling treatment from so many fighters, and to be honest I think someone like Alves who bothers to use leg kicks would beat him up standing too.
GSP, Fitch, Alves, Kos, Shields, Rumble, Stun Gun...
I'd fancy all these guys to beat Diaz quite comfortable. There's probably more too.