Because presumably (I don't have the new XCode to check) you can do things like:

    [someObject dictArg:@{ a:@"B" } arrayArg:@[1,2,34]];

So if they were both using "{" to set them off the parser doesn't know which one you mean until it hits the first ":" (and it can't guess from the type when the type is "id"). Now that's not impossible to write a parser that looks ahead to figure out what you mean, but it's much easier to parse if they have completely different syntax from the outset. And the "{" vs "[" distinction is very standard amongst the higher level languages, even if it isn't in straight C.