Why need the full blown type signature?

typesig.md

I have the following value and its signature

type JsonResponse = Affjax.AffjaxResponse Json
getUser :: forall e m. (MonadAff (ajax :: Affjax.AJAX | e) m) => String -> m (Either Error JsonResponse)
getUser user = liftAff $ attempt $ Affjax.get $ "http://someservice.com?user=" ++ user

Now I want to use that in combination with a Maybe. Easy enough just use map (or the alias operator)

applyUser' = getUser <$> Just "Bob"

Only this gives me the following error

82  applyUser = getUser <$> Just "Bob"
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

No type class instance was found for

Control.Monad.Aff.Class.MonadAff ( ajax :: AJAX
                                 | _0
                                 )
                                 _1

The instance head contains unknown type variables. Consider adding a type annotation.

in value declaration getUser'

where _1 is an unknown type
    _0 is an unknown type

Only when I add the signature as

applyUser :: forall e m. (MonadAff (ajax :: Affjax.AJAX | e) m) => Maybe (m (Either Error JsonResponse))

it compiles. I am a bit stunned as really I dont see why the compiler is not able to fully infer all data here