It's because Virtual 8086 mode is unavailable in long mode. So either you would have to switch the cpu in and out of long mode (which probably has a performance

It's because Virtual 8086 mode is unavailable in long mode. So either you would have to switch the cpu in and out of long mode (which probably has a performance penalty and complicates the code), or use the more general purpose virtualization support (which may not be available), or just emulate everything. It not surprising that Microsoft just decided that it wasn't worth it.