A portion of that could be related to lack of protocol support. 5k at 60Hz is more bandwidth that HDMI or DisplayPort could provide until very recently. Apple got around it by essentially making it two separate displays bound together in software using a single Thunderbolt cable, but that is only really feasible when you control the entire ecosystem.
Graphics cards that can output 5K60 over a single DisplayPort cable out of the box have been available since mid 2016. If anything, availability of HiDPI PC monitors has gone down since then.
