The novels I am currently revising for publication include post-apocalyptic science fiction and early medieval fantasy. For me at least, there is a logical connection between these sub-genres.
From the European point of view, the early Middle Ages was a time of enormous change after the collapse of the western Roman Empire: new peoples, huge population decline, rapid technological regression, epidemics, re-use of existing buildings and structures for new purposes, breakdown of previous law and order. I think it is interesting to imagine similar things in a post-apocalyptic world.
All of that I personally find interesting historically (and futuristically), but it also sets the writer free to invent and the reader’s imagination free – to imagine.