Yeah, pretty much what
>>4577 said. It was a lot of peoples' first exposure to mecha anime, so it was way different than basically anything else that was really available at the time.
Also, even though the "philosophy" presented in Gundam Wing is just typical AU Gundam-tier, "WAR IS BAD but check out these fucking rad fighting robots" stuff, it was really the first anime (and, in all likelihood, the first *cartoon*) they were exposed to that had any sort of message or ambiguity. It at least FELT, to us, like there was a lot more going on than "CAN THIS HERO BEAT UP THIS BAD GUY AND SAVE THE EARTH?!", y'know? Characters were talking about politics and maneuvering and stuff. That's something you didn't see in DBZ or Ronin Warriors.
Also, from my memories of my friends and I watching the show on Toonami, the fact that the characters carried guns around a lot helped. Aside from the fact that we were all edgy nine year olds, I think it really drove the point home that there was more at play than heroes in robots fighting villains in robots: They were at fucking war, and shit could go down anywhere.
But you're right, Gundam Wing kinda sucks and is entry level as fuck, and there's really no reason to watch it if you've got access to UC or 00 or something. It just had stuff that its competition at the time didn't.