I doubt licensing costs have much to do with it, it seems like flexibility would be the driving factor. Supercomputing centers, vendors, institutions which use

I doubt licensing costs have much to do with it, it seems like flexibility would be the driving factor. Supercomputing centers, vendors, institutions which use supercomputers, etc. all frequently modify and contribute modifications to the Linux kernel and associated userland. You have to imagine they are making changes which Microsoft could not afford to care much about, nor turn around in competitive time.