Why can't windows have linux/unix kernel?

Are you asking why MSFT can’t make a windowing system and toolkit to sit on top of a the Linux kernel? I don’t think that is impossible. What’s more complicated would be lower level such as permission ACLs.

There is no way to efficiently do this while still supporting the old Windows APIs. Basically all programs written for Windows would not work anymore. Instead, Microsoft is planning to ship a whole Linux kernel with Windows 10. But it will merely be an accessory to the main OS; Windows itself won't run on that kernel.