It seems to be the nature of the beast with Leica's lens offerings. Specifically with the Summarits, there may be the perception that they're "beginner" lenses with a built-in assumption on the buyer's part that they will eventually trade up to the "real thing". However I think a goodly number of people simply like to try "the latest" and find it isn't for them.
Within a year of the Wide Angle Tri-Elmar's introduction, I began to notice a spike in their used availability too. It's been similar with the 75mm Summicron ASPH lens (although many seem to find the the 75mm FOV is not their thing). Why I recently saw an ad for a used 24mm Summilux lens. That's a pretty quick turnaround, especially for a $7,000 (US) item.
That works for me. I love a healthy used market!