It's for two (well, at least two) reasons: one is to prevent things like this. The other is that in generally doing case-insensitive stuff for non-ASCII is language-dependent.

For example, in English "i" and "I" are case-insensitively equal but in Turkish they are not: "i" is case-insensitively equal to "İ" (U+0130) and "I" is case-insensitively equal to "ı" (U+0131).

So if you wanted to do full-unicode case-insensitive things you would need to decide what happens with <DİV> or <dıv> and whether it depends on the language of the page or what. Doing ASCII-only case-insensitivity sidesteps all that, while admittedly privileging English over other languages.