Because it broke backwards compatibility? It's worth noting that setuptools is in a similar situation to pip, where any change has a high chance of breaking things (as can be seen by perusing the setuptools and pip bug trackers). PEP 517/518 removed the implementation-defined nature of the ecosystem (which had caused issues for at least a decade, see e.g. the failures of distutils2 and bento), instead replacing it with a system where users complain about which backend to use (which is at least an improvement on the previous situation)...