Then why aren't 20-40% of MBP sales matte? What does a self-selecting survey prove?
Because Apple only sells the antiglare Macs through their own channel. No other retailer or online store sells the antiglare MacBook Pro. Even in the Apple Retail Stores, you can only get antiglare on the highest pricing tier for the 15" ($2349). If you want AG on the $1599 model for example, you have to do it online as a build-to-order option.
Back when matte was a no-extra charge option, the over whelming majority sold were matte. The "average Joe" doesn't really care, but he cares enough that if the price were equal, to go with matte. I have come across a lot of people who say "if I had to do it again, I would have gone with the matte." I myself said the same thing on a previous MacBook Pro of mine. Glossy was a new thing to Apple at the time and I thought it looked cool, then after six months or so, I started to regret it. Besides, antiglare just has more of a "pro" look.