cuz robb just wanted to be king of the north. renley/stannis wanted to be king of all seven kingdoms.
It didn't come across that way on the show. I read the books so i won't say anything. But i'm just going to go off the show. And in the show it looks to me like he didn't really want to be king of anything. He wanted to get the mothafuckas who killed his daddy. Waaaaait. Isn't that what Catlyn was doing up at Stormsend in the first place?