I think San Diego would have much diversity in entertainment, and like the areas mentioned by the previous poster are places that are widely heard of so would carry the latest trendy whatevers. Well known places need to so when people visit they can attract all sorts of sales from customers. . .
Well, Grand Rapids, Orlando and Palm Beach, I'd agree that they are widely heard of. Valdosta, Georgia isn't. Valdosta only has a population of about 40,000+, with just a little over 100,000 in the entire county, with estimates of 25,000+ being military and retired military. Not exactly what I would call a hotbed of gamer saturation. It's also smack dab in the middle of the Bible-Belt. Also traditionally not a hotbed of gamer saturation.
By comparison, the Metro area of Grand Rapids has over 1.3 million. Also Grand Rapids isn't exactly a Wal*Mart city. They do have some, just a handfull I think, because Meijers is king (Grand Rapids is their hometown). Which I'm sure limits there warehouse space, as you mentioned, yet the minis are there (and I'd bet so are the books considering The Little Ravens experience). Grand Rapids has about 3 or 4 dedicated gaming/comics stores.
Up until about 3 years ago, Valdosta only had one Wal*Mart, and it wasn't a super center. They now have a supercenter but the other store is declining. I wouldn't be surprised if they end up closing it entirely (the new supercenter is only about 5 miles away). There are also no other competitors in Valdosta (no K-Mart, etc.), and yet they had the minis. I even used to buy my Pirates strategy card games there (and this was at the original store, not the new supercenter). The original store was well away from the interstate, negating it getting a lot of travel traffic (the new supercenter is right off of I-75). Also, Valdosta only has the one RPG store (and for a year or so didn't have any), and I'm pretty sure it's the only game store in the county (for RPGs anyways, there is a CCG/Comics store).
I'm not doubting that Wal*Marts near you don't carry it. I can't speak to that so I'll take your word, but for everywhere else I've been, your statement just doesn't hold up. An I'd be willing to bet others throughout the country could say the same as me.
Edit: Not saying that I've been everywhere (a sheer impossibility), but I've only just retired from the military, and I can say because of being in the military, I did travel around a bit. Now I'm not saying I went looking for DDM minis in every city I've gone to, but I've seen them in a lot of varied places. And not all of them metropolitan, gamer saturated areas.