Another reason is that Unix™ is a registered trademark of The Open Group, which is, to use their own words, "an international vendor and technology-neutral, not-for-profit consortium." Unix™ is defined by them as an operating system which they have certified as hewing to the Unix 98™ standard. As I doubt that Linus Torvalds or anyone else has bothered seeking such certification, it's actually illegal to refer to Linux as Unix™. However, for all practical purposes, you need not give a shit.