Good question; here's an answer.
For the same reason you can't buy fine jewelry at WalMart; putting it for sale there misses its target market.
1) Products are neither good nor bad; rather, they either fit the intended use, or market, or they don't. Most shoppers in WalMart wouldn't have a clue how to use Menzerna Intensive Polish, to pick an example. For those people, IP would be a bad match of product to intended use/market. For Joe Average, a decent cleaner wax (Mother's, or Meguiar's), or a long-lasting sealant (NuFinish, Liquid Glass) fits perfectly.
2) Manufacturers pay retailers for product placement. It costs HUGE amounts of money for shelf space at WalMart, or Pep Boys. When taken in conjunction with the above statement (the fact that the more specialized manufacturers/remarketers won't hit their intended target), it doesn't make any sense at all, economically, for them to place product there. They would be paying to fail; that's absurd. Even Meguiar's only places carefully chosen products in big box stores; the "A" series (maroon bottles), Gold Class, and now NXT. You won't see #83 showing up there soon.
And, in the larger scheme of things, Meguiar's (along with Mother's), is an anomaly; they are actually a small company, family owned, competing with DuPont, Proctor & Gamble, 3M, General Mills, and the like. That their products are available at all on consumer's shelves is a tribute to the products' quality and customer demand; enough people had heard about them, and wanted them, and asked for them. At that point, it made sense for Meguiar's to spend, and for the big box stores to sell the shelf space.
I'm sure there's more, and that some people can add to that, but it's a start.
Tom