Because the waitlist system seems to only scan for available rooms periodically throughout the day. I'm not sure if anyone knows the exact times of the day it does this. So, as people are stalking the availability and a room comes up, they grab it and then when the WL system starts it's search later in the day, the room is already gone. It's a sucky system as far as I'm concerned. You have to look out for yourself.
Dean or Carol, please help me out here if I didn't explain that correctly!