As I understand it, it's because fixing them involves passing around a dictionary along with the data, and you can't do that with a standard declaration (it amounts to an extra chunk of data that's only *sometimes* wanted, and that "sometimes" complicates things). GADTs already have to pass around extra data in order to support their constructors and destructors; and, being new and not part of the standard, they don't have backward compatibility or standards compatibility issues, so they can get away with including the extra dictionary without breaking existing programs.