Because Labour are doing well in the polls under him?
They're consistently trailing one of the most incompetent UK governments in history.
See Big Abraham's post
Erm being 'even' with one of the most useless and divided governments in living memory isn't anything to write home about.
It's true that a Labour party lead by someone like Corbyn is not going to get anywhere near the leads Labour were getting in the polls in the mid 90s in the dying days of the Major government because he's never going to appeal to disaffected dyed in the wool Tory voters in the way Blair could. However you would have thought they would at least be able to open up a consistent mid to high single digit lead against a government as bad as this one.
Obviously things could change when something is done re. Brexit whether that be no deal, a second referendum, article 50 extension etc. but for left leaning people it should be at least somewhat concerning that Labour hasn't broken through.