I think in the case of the back-scabbard there's less documentary evidence (though I am by no means an expert) and people tend to gravitate to whatever works.
Historians like re-enactors for this reason. I can read up all I want and research as many archival documents as are available to me, but it take someone actually putting things into practice to see what works. Humans are pretty much unchanged from the Medieval period, so if a modern human finds it impossible to (say) draw a longsword from their back, then it is very likely that people would have found the same thing in the C15th.
