SPJ and probably many others are actually employed at Microsoft research centers. It looks like Microsoft just hasn't been able to find a suitable spot to push Haskell. Haskell influenced F# because they needed a functional language that targeted CLR, and included OO and mutable data.

IMO Haskell is even better than their languages

Maybe so but singularity actually provides the whole os apis via clr interfaces compared to mainstream windows os where the underlying apis are all in C, C++ and COM. The common intermediate language is not tied to any specific programming language such as C# or VB, it's more generic than that, and has it's advantages. Safety is something they wish to achieve but afaik their main goal is to write an OS in managed code.

Haskell does provide a safe runtime but afaik unlike the clr it's tied to the haskell language. I think there has also been some attempts to write an OS in haskell too though, but that's another story...