There are many reasons an excursion can be cancelled, but the most common is lack of interest; if a tour dosen't have enough response to be profitable, or if there have been a lot of negative comments in the passenger comment form, they will drop it. Or occasionally some natural disaster; one of the tours we wanted to take in Hawaii last April had to be cancelled due to landslides.
The wait list just means the tour is fully booked and your name will be put on a list in the order the requests are made and if people cancel, you will move up the list. If the wait list becomes long enough they will sometimes add additional departures--but there is no gurantee of that. If additional departures are added you will be notified once onboard.