Please note that the following is merely my opinion, based on my own observations, but...
I've seen nothing to lead me to believe that quality is a significant factor in deciding who goes on the main site and who goes on the Edge. It's all about sales (even the apparent exceptions). As far as I can see, those publishers remaining on the main site are of two types. There are those whose sales currently meet an arbitrary minimum sales level, and those who have shown a strong and steady increase in sales during their time with RPGnow and exhibit the growth potential to meet that minimum level in a relatively short time. The minimum sales target may well vary from category to category (niche products perhaps not held to the same numbers as mainstream products), but it's still all about sales.
The only place quality comes into the equation is in the general tendency for better quality products to get better sales than crappy ones (which is unreliable at best, given that factors like marketing, size of the publisher's catalog, and niche vs mainsteam products, will have far more impact on sales). But the split is not being based (so far as I can tell) directly on quality at all. There will be some products on the Edge that are better quality than some products on the main site, and the reverse will also be true. Any claim that a publisher's presense on the main site is somehow a guarantee of top quality, or that Edge publishers are necessarily of lower quality, is simply false. And any perception of such is flawed.
That said, there's nothing wrong with using sales as the criteria. It's actually much better than trying to use subjective "quality," since everyone has their own ideas about what constitutes "quality." Whether or not a particular publisher is "quality" cannot be measured objectively. How well their products sell can be.
So, that's my 2 cents.