The fans don't connect with them. I'll explain, whether most like it or not, there's a reason guys like Conor, Ronda or GSP for example got popular, and is not skill and in-octagon accomplishment only. Is because of how easy the majority of the UFC fans (the most important ones, the ones that make the numbers, even casuals, not exactly the hardcore fans) can connect with them and feel represented by those fighters. You may not think it matters but it does, especially in the "promoting" aspect. This is called Representativeness. For example, a Brazilian fighter is easier to be pomoted for Brazilians if he has the usual "favela hero" lifestory, people down here buy that easier, and will feel "represented" by that fighter way more than, let's say, Demian Maia or Vicente Luque. UFC can try their best to promote Nunes or anybody else, it will not work if she can't connect with the majority of the UFC fans, which i can really tell anything about. I guess is your average american ? idk. but yeah, is not that easily. Nobody is going to be breaking records all of a sudden just because they're more popular now. It's a rollercoaster situation, in case Nunes loses next big part of her newly acquired fans will leave. Ask Holm. TL;DR : Is not about how much UFC promotes fighters, is about how much they're accepted by the majority of the fans. Fans want to be represented by a fighter that is closer to their reality and even cultural background.