Because Ed Miliband suffered exactly the same and yet was polling much higher than Jeremy.
Covered in the report.
As the quote of Miliband Sr. at the outset of this report already pointed out, this is not an entirely new phenomenon in the UK and has happened before in relation to other leftwing leaders from Neil Kinnock to Ed Miliband (see Curran, et al., 2005; Gabor, 2014), but in the case of Corbyn the degree of antagonism and hatred from part of the media has arguably reached new heights.
So again, you're okay with a media that acts as an attack dog of the elites as long as they're attacking Labour leaders you don't happen to like? And yet you resent the modern Labour party's reputation as Tory-lite...