The press & establishment rounded on him, because he's too much of a threat to the established order. The nonsense about left-wing extremism, is laughable. Corbyn has never been 'hard-left', he is now as he's always been, a mainstream social democrat in the European tradition. Many of his policy ideas are very popular if you present them to people without reference to Corbyn. Many of them are standard practice in places like Merkel's Germany, but nobody is calling her a Marxist revolutionary.
Frankly, I'm sick of the lies, misrepresentations, 'alternative facts', and so on, spewing from the mouths of politicians, political commentators, journalists & the mainstream media. I expect our politicians to tell the bloody truth, and our press to hold their feet over the fire when they don't. The press should be holding the executive to account & speaking truth to power, not colluding with them to help maintain the current neoliberal status quo.
The people were mislead & lied to over Brexit, following decades of lies & misrepresentations from the Tory press, with claims of straight cucumbers, British sausages being banned, and more nonsense than you could shake a stick at. We've been fed a diet of anti-European trash, and fear-mongering over immigration, all to serve the interests of a minority of politicians & press barons. It's bloody shameful.