They probably started with quoted string literals (same syntax as ActionScript, I think, plus many other languages of course), then added string interpolation l

They probably started with quoted string literals (same syntax as ActionScript, I think, plus many other languages of course), then added string interpolation later and realised they needed a different syntax for it. (Another option: they looked at what the POSIX-style shell does, and did exactly the opposite. And as language design strategies go, that is far from the worst possible.)