It's his attitude and the way he carries himself more than his playing style. I admit he is sometimes electric, and just as effective as Ronaldo was, but he's far too inconsistent for a player that as you say, is well established in the league now. It's time for him to step up and provide consistent top class performances, he isn't a youngster that we can keep giving another chance to.
I genuinely don't believe Hernandez was fit all season, hence his continued non selection even when the Rooney Welbeck partnership went very flat mid season.