Because of the infamous hysterical raisins, I think. The achievement is all the more impressive if you know the history of KiCad. It has been evolved from loosely connected independent parts (even independent applications?) to a tightly knit application with several parts but one workflow. One big change happened for version 6 where the UI paradigm and the implementation of the schematic editor was totally changed. If you are interested in details, you can read for example KiCad 6: Just published my BIG *preview* review - #44 by eelik.
It’s probably impossible to make such evolution and revolution happen without some old incompatibilities left. I can’t imagine how any commercial software package could have done better – what I have seen, a large multifaceted application will end up as a mess, or it will be started almost from scratch and millions of dollars is spent on the development.
But another reason may be that the purposes of the editors are different and they have different internal workflows. It’s very difficult to map all shortcuts one-to-one when the functionality actually differs.
The third reason is probably that every time a small detail in the established workflow is changed, old users get disturbed and react negatively. Recently somebody complained why hovering over an item and pressing D doesn’t copy and paste (duplicate), like it did in v5 (see the big v5->6 change).