Not sure if it's been mentioned, but if the Wii is selling much better than the PS3, wouldn't it make more sense that people would want to try the PS3 if a PS3 kiosk happens to be working at all?
I wouldn't want to go to the trouble of giving my ID, etc., to play a game I may already have or easily could on my own Wii at home. A PS3 is more of a rarity, much more expensive, etc., and even though I wouldn't buy one for numerous reasons, I might be curious enough to stand in line for a few minutes to play one.
(Then again, at E3, I waited in line at Nintendo's booth for a couple hours and didn't even bother with the fifteen-minute line for the PS3s.
I heard the Wii line topped off at about five hours, the longest line at any E3 ever, while the PS3 lines were like half an hour long.)
Don't forget that there are many gamers that still love Sony, that non-gamers that might be interested in the Wii are still less likely to browse through a game store, and that some people are still insecure to be waving a controller around in public.