Am not the parent, but RPi is the only ARMv6 target of any significance; supporting an outdated architecture has non-trivial costs, which is why Ubuntu and other distros dropped the support.

It ships custom kernel and promotes doing things in a different manner that we are used to with Linux - boot.txt, overlays, rpi.gpio (even different gpio numbering scheme), raspicam and so on. Some of these are meant to make things easier for newcomers but IMHO it makes stuff much worse as you have a "special" rPi environment instead of standard Linux on ARM board environment.

You can run mainline on rPi just fine, with standard tools - the only problem is that most rPi projects on the internet won't work as they target Raspbian instead of Linux.