I'm guessing they only used it 10 years ago when it was very rough around the edges. It didn't integrate well with the old .NET Framework because it conflicted with how web.config managed dependencies and poor integration with VS. It was quite bad back then.. but so was .NET Framework in general. Then they rebuilt from the ground up with dotnet core and it's been rock solid since

Or they just hate Microsoft, which is a common motif to shit on anything Microsoft does regardless of the actual product.