The problem is that "Unknown" is so much less useful than None. Date of Death is "none" is much more of a useful concept when I'm building a database of people than Date of Death is "unknown". I have a lot more alive people in my database than we-don't-know-if-they're-dead people.
If SQL servers had algebraic types and I could roll my own "Maybe", then this bizarre decision would work. But they don't, so it doesn't.
Null can mean whatever you want it to mean. It can mean not dead yet if you want. And the logic of NULL works in that case too—two people who haven’t died yet don’t have equal dates of death.
The problem with having a value called “none” is that it would be tempting to make none = none return true, which is madness.
(I do agree it would be nice if we could have multiple kinds of null instead of just one. But I would want them all to have the same equality properties as NULL.)
If SQL servers had algebraic types and I could roll my own "Maybe", then this bizarre decision would work. But they don't, so it doesn't.