many options and ignorance
I just found this site about a week ago, since I had mistakenly invested in a timeshare (but rescinded in time). Before that I had NO idea that these timeshare resales or rentals even existed. I'm not young, and one of my parents also had a timeshare.
Now people have so many options to find lodging--it used to be just hotels. This doesn't necessarily explain year to year differences you are experiencing, but with Airbnb and all these cheap travel sites, the time share rental market is not as visible. Just in the past year I have met or found out about many people using Airbnb. It seems that it is the "go-to" for most of the under 30 set, and particularly for short term the majority of people I know who are not wealthy, seem to check first at Airbnb. I don't use it and it doesn't appeal to me, but I'm intrigued by the timeshare rentals and will be checking those from now on.