4) There isn't a universal anything. There's no base system for developers to depend on, there's no consistency to the interfaces, widgets, clipboard behavior, or anything else.
5) Pretending there is no line between "system" and "application" and relying on a package manager and repositories is a terrible paradigm with a long list of problems roundly dismissed and ignored by the community.
6) The community is terrible. If you have any issues you'll be told you're using the wrong distro (no matter which distro it is), you should read the manual (even if you did), you don't really want to do that (I do, actually), "normal people" don't do that (I care why?), and finally that the source is available and you can fix it yourself.
7) The whole system is a gigantic Rube Goldberg machine of disparate components developed largely in isolation and slapped together by whoever decided to build a distro today.
And probably about a hundred other reasons. But rather than try to solve any of them the Linux desktop community prefers to pretend that they don't exist, and the REAL problem is that Microsoft is a bully and won't let companies install Linux by default or something.
This guy maintains a more comprehensive list: https://itvision.altervista.org/why.linux.is.not.ready.for.t...
To be fair, some small progress has been made and Windows is currently being sabotaged by Microsoft, so Linux Desktop may yet win by default.