Because Cyborg simply is not a star outside of hardcore MMA fans. And she's not a "star" among us, either. She's incredibly divisive, has already turned down a contract with Zuffa that would have allowed her to work her way down to 135, and submitted the biggest joke of a "doctor's note" in MMA history.
"If I try to make 135 I won't be able to have kids."
Let's be adults, brahs. If she doesn't lay off the steroids and stop powerlifting like Sister Steele, then she won't be able to have babies. She won't get to 135 because without her size advantage she is NOTHING.
Cyborg's technique is extremely lacking, both on the feet and on the ground. She fights like a female Wand, which works against 99% of WMMA competitors. She is a below-average grappler, and was twice mounted by Gina Carano. She has no submission game, and relies on that pharmaceutically-enhanced power to get out bad situations. On the feet, she lacks true one-punch KO power (something that even Ronda has shown), and finishes her physically overmatched opponents with flurries of strikes that leave them unable to defend themselves.
Furthermore, a flurrying opponent with a subpar grappling game is tailor-made for Ronda. Do you guys think Cyborg is magically going to develop some counterpunching, circling-away style like Machida in order to defeat Ronda? The VERY FIRST TIME Cyborg comes forward with her "punches in bunches" she's going to get hip-tossed into Bolivia. This may take place several times over the course of a fight, but I don't think Ronda would need more than a round to dispatch Cyborg.
SANTOS KNOWS THIS. All the female talent is at 135, but a very small, yet vocal, minority of MMA fans are clamoring for her to be in the UFC. At what weight? 145? Who would be in a 145-pound division? The second-best 145-pound fighter in the world is 0-2 against Cyborg and is back at 135. Cyborg's entire strategy is very simple, and it surprises me that educated MMA fans are falling for it. Cyborg wants to come to the UFC at 145, squash a couple of fans, and hope for a big payday catchweight fight against Ronda. If Cyborg wanted to make 135 and compete for the title, THE TITLE FIGHT AGAINST RONDA WOULD ALREADY HAVE TAKEN PLACE.
This is never going to happen. Ever.