kde was nice but not "open source" so gtk and gnome came and drove lots of contributors. i think it was around kde3... then Ubuntu and later google dumped lots of money on gnome and some designers just stole the leadership it became a race to the bottom with copying all bad ideas from apple UX. then Ubuntu parted ways and did it's own thing. then they merged and rewrote the thing in JavaScript and release without even a settings application working... and then contributors mostly moved back to kde and 5 was great. now 6 is polishing everything and building on dbus everywhere and designers already started the UX capture but doesn't seem it is going anywhere.
anyway, i don't have much reference material for the time most contributors moved from kde to gnome in the name of GPL (noble goal that wasn't used since distros didn't abandon the broken release for the mint fork when they should have)