Comment by pjc50

Firstly a strong desire to self-host: write the build system in the language itself.

Secondly, often very differently shaped requirements. The dotnet SDK tries to keep its build specification (.csproj) files editable by Visual Studio, which is why most of the stuff in them is just XML properties.

You probably could build C#/F# with Bazel but that's not what Microsoft chose, and you kind of need to stay aligned with them and the large amount of MSBuild files in the SDK.