There are two reasons a trip is listed as waitlist
1-A private group has it and if they can't fill it by a certain date (whatever their contract is with
ABD) then it is opened up to others.
2-ABD is holding the date and if they get enough interest they will make the date available.
I have never heard of anyone who was on a waitlist get the a trip however most posters on this board book a date that is available.
Good Luck