Comment by abecedarius

That ship already sailed when Python 3 made comprehension expressions (like list comprehensions) introduce a nested scope. They're even a kind of 'for' too. (Previously, [x for x in y] would clobber any x in the same function. Fixing this meant implicit nested scopes.)

It's true that this would've been a further change (every loop entering the implicit nest, rather than entering one nest which then mutates per loop). But this design was a decision, not a necessity. JS's "for let" shows this by example.