"F# missed by Roslyn" - it's about tooling rather than the perfect academic compiler language choice. The best language to write a compiler in is the target language of the compiler.
The best language to write a compiler in is the target language of the compiler.
Citation please? Writing a compiler in the language it's targeting is not always "best", but it is often done to reduce dependence on external toolchains and to flush out bugs in the language or compiler itself (since a compiler is usually a fairly complex program).
Every language has it's own benefits and drawbacks; the ML family of languages (which F# belongs to) was purpose-built for implementing compilers (and theorem-proving tools), so it would have been an excellent choice for implementing Roslyn, or at least parts of it.
If you're writing a general purpose language, the benefit in dogfooding is impossible to beat. Language design and implementation are close bedfellows, and having the language design directly affected by the language is practically invaluable.
