Tate was a WBW punching bag and destined as a journeywoman competitor until the right circumstances (Rousey out of picture) got her a title.
Rousey is an American Olympic medalist, won fights dominantly, was able to play both heroine and villain personas, became and defended the first UFC women's title.
She had everything and made the most of it. Holm proved to be a fluke (she's 0-2 going for a FW title), Tate was a fluke and really just a gritty fighter, Nunes is still an unknown but she's clearly not marketable in the sense of: (American) Olympian, personality, looks, etc.