I am not deeply sold on this idea as a whole, but it seems like having a very low-maintenance van has its advantages right now compared to a hotel room which has probably sat empty for the majority of the last few years. As you say, it seems unlikely that airBNB is explicitly pushing them at the expense of other listings; more likely, it just is a consequence of the world we live in.
[1]https://prtimes.jp/main/html/rd/p/000000586.000030117.html (Detailing a niche hotel where you get to pretend to operate a main tokyo train line and be woken up by the same device train conductors use)