I think a lot of people overplan their trips and have to have ADRs for every meal. Then when they are there they realize they really don't want to eat in Mexico, France, etc. or they are in line for an attraction and don't want to leave. Could be many reasons.
A year ago May, we had reservations at Brown Derby for 5. Tried to change it to 7 to no avail. While eating we noticed a lot of empty tables including some large tables(and they were turning away walk-ups). Our server said that it was happening a lot and by the time they realized people weren't going to show, the next set of reservations were showing up.
I always cancel ADRs we aren't going to use and wish more people would do the same.
Cyn