x11 had long time become unsustainable in multiple ways
braking changes where inevitable
all of the most relevant DE had somewhat already gutted out X11 leaving nothing behind then the interfaces
so the replacement being defined by interfaces and "every DE for themself" was just natural, for the large DEs it was already the case anyway
and for the other DEs you could say wlroot is now the common core they need because they don't have the will/time to implement everything by themself