Well, Ubuntu = Unity OS
most Gnome developers work for Red Hat, so Fedora = Gnome3OS.
KDE had no real distro favorite. Kubuntu was seen as one for a time a few years ago, but a lot of major developers left for OpenSuse or Arch.
Maybe if Kubuntu dies the Debian KDE team will get some such needed people and will eventually become the place to go for polished upto date KDE SC releases. Who knows .. pulling the plug on Kubuntu might be good for KDE.