I think the market doesn't have room for another desktop OS beyond niche uses.
As in, why would a mainstream user go with Linux over MacOS or Windows?
Same deal with Android and iOS. Insufficient room or need for #3.
Also, apps/software. Devs don't want to support 3 OSs. Users don't want to switch to an OS missing software.
If Microsoft couldn't do it with Windows phone, I don't see why anyone else could.
(Although, I just recalled, Chromebooks exist, found a niche (education, grandparents) and found a monetization method (vampiring on user's privacy.))
Makes me wonder if Ubuntu could do a ChromeOS-like strategy. But without the data-sucking.