AFAIK the main reason is in how windows handles the filesystem - in linux everything is a file and all files are cached by default unless that memory is needed by default, so 100% memory utilization is the norm and where Linux operates most efficiently. In windows file caching seems architecturally be an after-thought and much less efficient - i.e. this causes handling a lot of files (like when updating the OS, where a lot of files need to be modified) to break the caching system and cause a lot of cache thrashing.