Why can't I simplify this pattern match?

This works fine:

data Possibly a = LolNope | Yeppers a deriving (Eq, Show)

instance Functor Possibly where
    fmap f (Yeppers a) = Yeppers (f a)
    fmap _ LolNope = LolNope

Repeating the LolNope in the last line doesn't look elegant. Replacing the last line with the following doesn't work:

fmap _ z = z  -- error: Couldn't match type ‘a’ with ‘b’ ...

This doesn't work either:

fmap _ z@(_) = z  -- error: Couldn't match type ‘a’ with ‘b’ ...

Why don't they work, and are there alternatives that work?