a couple reasons it wont work.
1) the UFC IS the history of MMA. if you are the UFC champ, you are king. there is prestige in it. You cant tell a top boxer to forget that WBA/WBC/IBF belt that was held by Ali, Foreman, Leonard, Hearns, Hagler, etc........Try to get that UFC boxing strap! Its the same reason BKB failed. No prestige in it for the boxers.
2) you cant tell a top guy to sign and exclusive UFC contract where you are gonna make what they want to pay you, which will be much less, more than likely, than they can get on the outside. And Dana cant offer market value, because if he does that, his MMA guys are gonna go apeshit that they are getting 40k while boxers are getting 500k.