Um...because they get more money fighting elsewhere?
Roger Gracie is just about the only Gracie active in MMA these days that might be able to hang with the current competition in the UFC. Royce is one-dimensional, Royler and Rickson are too old, Ryan is dead, Renzo is semi-retired and affiliated with the IFL anyways, and Rorions sons are too young still, but might become competitors later.
And then you have to factor in that the japanese orgs pay a premium just for the name Gracie - regardless of the first name, while in the US basically just Royce could command equivalent salaries.
I do want Roger in the UFC flying the flag for the clan, but right now the closest thing we have to a Gracie in the UFC is Ricardo Almeida, a long-time student who is basically an honorary member of the clan
