I think the most important part of the OP's complaint is "blessed". For a very long time Mono was an entirely unofficial reimplementation of .NET for Linux. You can't blame anyone for being hesitant about relying on it.
How can an implementation of a standard (ECMA-335 or ISO 23271), which is even explicitly intended by MS to be used by others (even supported by a comprehensive MS "open source" implementation to illustrate it) be "entirely unofficial"?
Been a while since I’ve touched the CLR, but I have this vague recollection that while the Mono runtime is blessed, it’s also incomplete. If you want to write a portable CLI you can use Mono, but if you want to show a GUI, you’re out of luck.
Mono included a powerful GUI framework which run on far more platforms than the .NET GUI. And Ximian/Xamarin invested a lot of effort to be as compatible as possible to all aspects of .NET (see e.g. https://www.mono-project.com/docs/about-mono/compatibility/).
Because the standard wasn't enough to build a complete app - parts are (were?) missing.
It was also not explicitly intended, blessed or even recognized by MS for a number of years. It was just another open source project.
This gave rise to other platforms that were attempting to improve on C (GNOME) / C++ (KDE), such as D, and finally JavaScript for GNOME. Meanwhile, web apps (and then Electron apps) became de-facto defaults due to their cross-platform nature.
Had MS embraced (err...in a positive way!) Mono from the start, taking into consideration Miguel's influence in the GNOME community, GNOME 3 might have been written in .NET.
The standards cover the CLR and core library, i.e. the stuff which is replaced in case of an AOT compiler. The .NET framework is an implementation on top of CLR and the core library, and the Mono project included a version with the intention to be as compatible as possible to .NET, but also an alternative, cross-platform framework, which was/is suitable to build all kinds of applications, including GUIs. Concerning the "blessing" I don't understand why people are less concerned with JDK than with Mono, because the owner of the technology is not exactly known for not enforcing his rights in court (see e.g. https://en.wikipedia.org/wiki/Google_LLC_v._Oracle_America,_....). But fortunately there are limits to copyright when it comes to compatibility.
