The wheels started to come off with .NET back when ORMs became a thing, slightly before Rails. Look up "ALT.NET" movement for the history.
TLDR: Microsoft wanted .NET developers to only use tech from Microsoft, used various tactics to achieve that, and people got the message. If you wanted to use modern tech or develop innovative stuff, .NET was not a comfortable place to be.
Microsoft have tried to move past that, but never really have. .NET has never had a healthy Open Source eco-system.
TLDR: Microsoft wanted .NET developers to only use tech from Microsoft, used various tactics to achieve that, and people got the message. If you wanted to use modern tech or develop innovative stuff, .NET was not a comfortable place to be.
Microsoft have tried to move past that, but never really have. .NET has never had a healthy Open Source eco-system.