E J that is fantastic.
I think there are many obvious reasons for the low-count used backs in the market. Some have already been mentioned. Most amateurs who shoot these backs either have a day job or are retired. Every amateur I know who has a back also has another smaller-format camera system, either DSLR or mirrorless. All these add up to the back being used less frequently than pro use.
The back I traded in had something like 11k actuations, which is probably in line with most of my other cameras. The difference is I used that back for 5 years! Not many shots per month.
I've had the 3100 now for about a month with zero opportunity to get out and even less confluence of time with good light. So far only one mediocre opportunity, and the light turned out to be a bust.
Edit: 2-image stitch, shifted 18mm each way, sk60xl, Alpa STC, 1 deg tilt.