My thoughts...
Ronda Rousey wanted to take some time off and wanted to get another fight in before she does. To be the co-main on UFC 175 is the only one that work out for her. UFC 174 and 176 dates would not work. Then there is the probelm of moving another title fight to another PPV card. UFC 177 should have been Rousey vs. Zingano, but Rousey wants that time off and Zingano needs a tune-up/warm-up fight.
Another thougth is that they will make the the co-main event anytime Rousey has an opponent who would not be a draw, or has no chance of winning. It was also hard to keep Rousey vs Tate II as a main event, when everyone knew Tate was not deserving of a title shot, rather one was gifted to her.