From what I've heard from KDE devs wrt similar queries, the usual response is "Standards Compliance". There's apparently something in the window manager standards that KWin adheres to that doesn't accommodate having each monitor on different desktops. In my opinion the standard should be updated to accommodate multiple screens acting independently.

I've also seen a couple of topics about having different monitors on different Activities (with the usual case being a new/separate Activity for use with external presentation displays). I think there may have been some work done around that, but I can't say for sure. But if there is some work underway around that then that could be another potential solution. At the very least you can assign each window to one or more activities, or make windows "sticky" so that they appear on all desktops, which you may be able to use to accommodate what you want (though it isn't as flexible as allowing each monitor to be on separate desktops/activities).