As mentioned already he was maligned by the mainstream media (yeah the MSM!
and he never stood a chance since then. From the moment he stood for party leader the media mercilessly mocked his style of left politics. When he got closer to winning that contest it was literally day after day non stop humiliation, yet he still won the leadership contest. Ever since it's been a never ending drip feed of anti Corbyn, anti left bashing in the papers especially but Keunsberg and co on the tv too. Politics of the left, the non Blairite kind of actual leftist policies are deemed not worthy, silly, not viable. When people are told that day in day out then it's no surprise Labour have slid and slid as the electorate believe they're a joke. Self fulfilling prophecy, no matter what you think of Corbyn and how he's conducted things.. even if he'd been absolutely on point and not made one error Labour would still be sliding. Moderates who would actually agree with his vision of Labour now feel like the party is unelectable and too compromised to vote for. Why? Mainstream media (yeah that is a catch all term but pretty much the media most people see on a day to day basis, the vast majority of which is trending very much rightward).