It would make more sense to have conor fight masvidal. If he wins, they could use it to justify a title shot at welterweight--> $$$. If he loses, a nate fight would still make sense since he just lost to masvidal too --> $$$. Then if nate wins that, it would set up the masvidal rematch on some "we didn't finish last time" shit --> and still $$ (could do the same for conor depending on how the aforementioned masvidal fight with him went).
Whereas if he just fought nate and lost, there wouldn't really be any big money fights to make at 170 (relative to how big money any conor fight would be).