I don't have any experience with this, but I suppose it's probably due to a) a lack of awareness and b) a lack of support.

For one, people probably aren't aware enough of unicode to think to use a unicode character in their password, and they probably also aren't aware of how to actually type a unicode character.

Plus, website support for unicode chars is... unpredictable. If I set my password to "foo1234", then I expect it to be "foo1234", not just "foo" and a lack of any error. Similar to how password length is sometimes just randomly truncated.