I do think it's becoming a bit frustrating how many top fighters or prospects aren't in the UFC, at least from the perspective of seeing the best fight the best. But you have to remember the UFC is entertainment oriented. It's focused on excitement and storylines, not meritocracy or featuring the best fighters. You have to remember, this is what people want. And no, not just american people. The UFC caters to the western euro market, they try to market south and central america, they want a presence in asia, the UFC wants as much diversity as possible because the more diversity, the more markets. The UFC also wants entertaining fights. They want fighters who are...how to say this...very reckless and uncalculated. If you wrestle well, you're a wrestlefucker and the UFC doesn't want you, if you strike smart, you're a point-fighter and the UFC doesn't want you. And to be fair, like I love wrestling so I'm not a fan of the UFC's approach, but I do think most people agree with them. Most people prefer when guys swing wildly and walk into punches and kicks. Because it creates more wildness and action.
If you fight recklessly, you won't have a long career, and even if you do then not without suffering a good bit of brain damage. Fighters who want longevity play smart, but the trade-off is that you sacrifice something in entertainment value because you will be fighting to minimize risks. The UFC doesn't sign the best, but be honest, do you really want to see the best? People complain all the time these days about GSP, well GSP was the best. He minimized risk, consistently outscored his opponents, and now people don't like him. And if you don't like GSP, you're going to convince me that you'd be hyped to see Makhachev vs. Mamedov? Of course not. Even if Mamedov came to the UFC and then UDed say Riddell, RDA, Hooker, Chandler in quick succession and the fight was the title fight to make you wouldn't want to see it.