The only full-frame image sensor I'm aware of that you can buy in small quantities off Mouser and the likes are the CMV20000 CMV50000 (20 and 47.5 mpix) costing $2000-$4500 per sensor. Last I checked APS-C and especially M43 sensors were 1/4 to 1/10 the price.

So imo it boils down to price.

If I had $1M to burn, I'd love to make the same product in full-frame - with Leica-style microprisms so that it would work well with range-finders. Maybe now that film is getting really pricey, maybe there's more of a niche to use old cameras with digital sensors.