Because it reached it's peak - when community interest was big, FOSS participation interest was big, even corporate interest was big, and desktop OS interest was also big - and didn't manage to fully catch up to the areas it was lacking.
It's still better in other areas, and of course, freedom and lack of surveillance, but regarding catching up to Windows and macOS in UI maturity and convenience, it's too late.
(And programmer/admin types, or people who "set it for their 70 year old parents and they use it just fine" are not a counter-argument).
Fortunately, macOS and Windows are going downhill UI wise too, so maybe they'll meet Linux further down, and Linux might even surpass them.