Couldn't be trusted on national security
Jeremy Corbyn repeatedly demonstrated that he could not be trusted to protect British national security interests.
On polling day, Labour’s core support base, working-class, northern manufacturing towns, turned their back on the party. In some cases, areas that had voter Labour in every election for almost a century made the transition across the aisle and put their cross in the Conservative’s box. Jeremy Corbyn, as a middle-class north Londoner, was utterly ill-equipped to carry the northern vote.
Labour has traditionally drawn support from two major demographics: the British working-class and university-educated middle-class voters. Despite differing in cultural attitudes, these two segments of the British public came together under the Labour banner due to their shared economic interests. The loss of either one of these support pillars would provide disastrous in an election. Jeremy Corbyn's inability to appeal to working-class voters was ultimately his downfall. As a middle-class Londoner, he was always going to be a hard sell to the Northern working-class. But going into the election campaign, Jeremy Corbyn was the most unpopular opposition leader in British history. He had a net approval rating of -60.  Not only was Corbyn unpalatable to Northern voters, but he also chose to fill his shadow cabinet with fellow middle-class Londoners. Diane Abbot, Emily Thornberry and Kier Starmer all came from neighbouring constituencies and suffered from the same challenges as Jeremy Corbyn in appealing to northern voters.
Canvassers in working-class Northern and Midland communities testify that Jeremy Corbyn’s name rarely featured in conversations on the doorsteps. As far as they are concerned, he is just another middle-class Londoner leading the party.  Gordon Brown and Tony Blair weren’t working-class heroes, but they managed to maintain the support of Labour’s fabled ‘Red Wall’. This indicates that it wasn’t Corbyn that was unpopular among working-class communities. He is just the latest leader of the party in a long line of posh politicians.
[P1] Corbyn was unappealing to Northern voters. [P2] The loss of the Northern Labour heartlands was the reason the Conservatives were awarded a commanding majority. [P3] Therefore, Corbyn was the reason the Labour Party performed appallingly in the 2019 General Election.
[Rejecting P3] Corbyn's limited appeal wasn't the reason voters turned against Labour. Gordon Brown and Tony Blair weren't desirable leaders to Northern voters, but they never lost Labour's "Red Wall".