Why did the fact that Robb won every field engagement decisively seem to count for anything with the other players in Westeros?

I think there might be a missing negative there, but I’m not sure so I’ll answer both ways. I think people did care – it’s definitely one of the reasons why Reny engaged in diplomacy with the Starks and why Davos and Cressen were urging Stannis to the do the same. 

At the same time, some people – like Balon Greyjoy – clearly didn’t care and were basing their opinions more based on pre-war status quo.