For me if we're going by his personal polling figures this ultimately just boils down to the fact that the public just...don't really like him. It appeared as if he'd reversed this in 2017 after a dodgy start in which he'd been vilified beyond belief, but the graph shows his brief popularity (if it can be called that) has basically fallen away again to the point where you've got someone who's been in charge of the main opposition party for four years while constantly maintaining negative polling. That's dreadful, however you want to look at it.
Yes, the media are dicks to him, but he hasn't exactly helped himself by basically not having a position on the single biggest issues that's impacted the country in decades. He's a solid campaigner when he puts his mind to it but leading an opposition party (or indeed one in government) involves having to do much, much more than that.
While he's arguably more toxic than Corbyn due to some of his past comments, I reckon McDonnell is probably more effective than Corbyn.