"there is not really a good technical reason why the vast majority of x64-clusters running Linux could not be converted from static Linux machines to dynamic Linux-Windows images"
Rather than look for technical reasons why Windaz ia not on HPC clusters, perhaps we should be pondering what a HPC cluster user has to gain from putting Windaz on HPC clusters.
If the answer is "nothing" then what exactly is the point?
I have no idea how MS licensing works for their so-called "Windows HPC server" (or whatever it's called) but even if they don't hit you with per core licensing you can bet your bottom dollar that MS stands to make a bucket load of cash from a 64K Core HPC cluster. Or to put it another way, the owner of the HPC cluster will have to give a bucketload of cash to Microsoft.
So again, I ask you, what possible benefit could a HPC cluster operator gain in return for that bucketload of cash?
Maybe they really want to code their stuff in VB.NET or something.