Because Cowboy is a big name that will sell well and Conor knows he is a guy that will stand with him. So in Conor's mind it's very winnable fight with a low chance of being dried hump/gnp to a finish or decision. He also seems to respect Cowboy as a fighter. Cowboy's is a fighters fighter more or less. The number next to Cowboy's name means almost nothing to Conor as like I said he could probably get a title shot if he demanded it. So he beat Cowboy he'll be looking for his title shot or whatever big money fight he can get.
P.S.: Conor seems very intelligent in knowing what the fans want or don't want. I think he knows if he goes throws himself right back into the mix at the top or title fight it's going to really set off a lot of fans. Not to mention a greater chance of a lost that could hurt his image even further.