Should a cyrillic `а` and a latin `a` have the same code point?
If they did then there's no consistent way to group those two alphabets, one or the other would end up with letters outside it's main grouping.
And what happens if the shapes of those two characters divege over time? Do you have a breaking change to introduce a new code point for one of the languages, or do you just make it impossible to have a font that can be used for both latin and cyrillic at the same time?
This isn't just a theoretical problem, by the way. There are characters in Chinese and Japanese that share a code point but the shapes of the characters aren't the same in the two languages.