Perhaps Hyper-V team is already tired dealing with Windows. I mean look at this quote: "...it takes approximately 16 hours to compile Windows on a 64 cores super fast server-class machine optimized for the job, and with hundreds of GB of memory, and that time does not include running tests." [0]

[0] https://www.quora.com/What-do-you-think-about-open-sourcing-...