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.)

Shell and basically every other language that makes that distinction (PHP, Ruby, Perl...).

This is pretty annoying, but you get used to it. After so long it would be a major pain to break everyone's strings by flipping the syntax.

IIRC, you can use either. Whatever quote you used to start the string is the one that will end the string.

var foo:String = "foo"; // foo

var bar:String = 'bar'; // bar

var fooquotebar:String = "foo'bar"; // foo'bar