I think it is because of the lack of characters present on the Strip. Everywhere you go it is described as being the best place in Vegas and the most populated; but unfortunately this is not the case. They didn't add enough people or random activities I think. It just feels empty.
I think that's it. I mean, I like the concept of the Strip, the casinos look nice and all that, but there's just not enough to do there. There aren't really that many quests there, and the only real side activity to do there is gambling, which you can only do until you're banned from all the casinos, after which, they're just decoration. Really, the most common reason I go there, outside of quests that require going to the Strip, is to visit my home in the Lucky 38.
I guess the problem is that it's justr kind of dissappointing, I mean, Obsidian kept talking about how impressive the Strip is, but really, once the novelty of seeing a place like that in the middle of a stereotypical post apocalyptic wasteland has warn off, you find that there just isn't really that much to do there. Granted, a lot of places in the game don't give you much reason to return once you've done all the quests there, but the main difference here, I think, is the importance the Strip is supposed to have in the game, and how much it was hyped up. I doubt post people expected anything really impressive from Primm, but the Strip, on the other hand, kind of just left people dissappointed.