It's the same reason internet direct companies tend to not get reviewed in print mags.
On the grand scale, they're better bang for buck, and B&M companies don't want mediocre value being associated with their products. When you factor in all the overheads and markups brick and mortar products go through, you can certainly compete with other B&M companies, but not companies that aren`t paying those overheads.
Companies like Aperion, EMP, Salk, Axiom etc may feel confident in their product against things in the same budget price ranges of sub-2000, but as soon as you get to Hi Fi companies like Totem, Paradigm, DefTech etc, you have to add a certain amount in order to make a direct comparision. These companies prefer to compete with each other because they know that for value, ID companeis likely have them beat.
I`m not saying it`s a be-all-end-all rule, but it`s safe to assume that this is why those companies don`t send their products to Audioholics. I`m sure a company like Klipsch feels comfortable selling its boom and sizzle speakers to a person who doesn`t know much about a live performance that`s used as a reference. In last year`s Budget Floorstander shootout, we saw a clear divide between the trained listeners and the non-trained listeners... and we also saw the JBL speaker lose out rather unanimously even if it`s a relatively nice product for its price in terms of B&M brands.
I don't think B&M companies want AH to review their products. It`s not like AH is going around into factories stealing speakers to review them like the Grinch.
In fact one of the more recent speaker reviews I read on here was this one, which AFAIK is a B&M brand:
http://www.audioholics.com/reviews/speakers/floorstanding/pinnacle-bd-1000