MMA is an entertainment business that just happens to be a sport.
Tickets, PPVs, merchandise, etc. are sold based on consumer demands. Historic trends show that business peaks on strong personalities and hot feuds (grudge matches). The fighting is somewhat secondary to this, and is just the content that is given in exchange for the interest. But if fighting was all that was needed, guys like Mighty Mouse and Jose Aldo would've had no problem being PPV draws.
However, some people just don't gravitate to the promotional aspect of the MMA business, and Sara McMann is one of them. McMann seems to be a competitor only, and is unapologetic about it. However, in addition to her lack of promotional skills, her fighting style is also the problem, as she is the classic lay-and-pray point scorer, which is also unappealing to the masses. If she were the one to dethrone Rousey, it would be disastrous for the UFC, as she's unmarketable in every single aspect. But losing to Tate was a major setback, and she needs another three straight wins to even be in contention.
It seems like everyone who Rousey crushes seems to go into instant denial and demands a rematch, with Tate being a notable exception. Good thread, TS.