Curious, why not?
Because if i understand correctly it's designed so that Apple cannot see any of the data...
So for what reason would you want to provide another third-party provider, but not Apple Signin?
To me it seems a huge win for consumers privacy wise, and Developers have nothing to loose.
not a win for privacy, but i'm sure there are some that would be unhappy about not getting the data the other providers are giving them.