[Haskell-cafe] Why were datatype contexts removed instead of "fixing them"?

If I understand correctly, the problem with datatype contexts is that if we have e.g. data Eq a => Foo a = Foo a the constraint Eq a is thrown away after a Foo is constructed, and any method using Foos must repeat Eq a in its type signature. Why were these contexts removed from the language, instead of "fixing" them? PS This is following up on a discussion on haskell-beginners, "How to avoid repeating a type restriction from a data constructor". I'm interested in knowing whether there's a good reason not to allow this, or if it's just a consequence of the way type classes are implemented by compilers.