I think the answer is: because Microsoft let it. I'm a big fan of modern .NET, but my biggest complaint is that Microsoft views, and always has, the CLR as the C# Language Runtime and not the Common Language Runtime.

For example, see the relationship between F# and C#. The CLR is constantly getting features that are only to support features in C#, leaving F# in a position where they either don't get the feature, can't add the feature, or begrudgingly add the feature to keep up compatibility with C#, which is something it does take seriously. But this has the effect of "dirtying up" the F# language by either adding features that don't really belong in the language or keeping features out.

The other thing is that C# consistently adds features to itself that are inspired by F#, since F# already implements these features on the CLR, thus showing their viability. So what happens is that C# continually approaches a more bloated language with a subset of it being a poor copy of F#. But then F# gets dragged along towards having a small subset of C# in it for compatibility purposes. So it's simultaneously making both languages worse.

Even the iron languages project that lead to IronPython, IronRuby, etc. was a bit of a Trojan horse to test out and exercise the CLR and .NET with no intention of ever providing long-term support for those projects. The DLR, which was implemented to support those, appears to be just maintained by a skeleton crew of people invested in it, probably by those interested in keeping IronPython up and running.

I do not understand why Microsoft takes this approach. It is myopic, shows a misunderstanding of their own technology in the CLR, and ultimately turns C# into another C++, leave dead languages and projects in the wake.