It's because by not being mediagenic and still getting voter registration without their blessing he makes them look like the out-of-touch, biased fools they are.
He takes back the power they insisted on controlling, making them come to him now so they don't come across as even more irrelevant.
Media has been used to influence old voters to vote for middle-aged politicians.
Now young people sign up in droves for an elderly man, in spite of the MSM's best efforts to smear him into oblivion.
The increasing terrorist attacks make her look weak, her efforts to diminish Corbyn make her look petty and unsportsmanlike, and she has no disarming charm to elegantly play the overdog, reverting to rabid at every opportunity, (when the public love the underdog).
Her cowardice in not debating Corbyn flies in the face of her 'strong' image, and her wobbly meltdowns when interviews have gone internationally viral, making the 'stable' pitch ludicrous.
I wish Corbyn would attack her for her flagrant conflict of interest with her husband's financing arms deals which she helped broker, and her appalling handling of Libya as Foreign Secretary, (the shenanigans with the 'Manchester Boys').

Then we'd see some serious sparks fly.

'The history of public debt is full of irony. It rarely follows our ideas of order and justice.' Thomas Piketty