Couldn't be trusted on national security
Jeremy Corbyn repeatedly demonstrated that he could not be trusted to protect British national security interests.
Labour has faced allegations of rampant anti-Semitism within the party for the last three years. Several prominent Labour figures, including Ken Livingston and Chris Williamson have been suspended over anti-Semitic comments and Jeremy Corbyn has shouldered a lot of the blame for failing to take a stronger stance on eradicating anti-Semitic thought within the party.
During a television interview with Andrew Neil during the election campaign, Corbyn refused to apologise to the Jewish community for his inability to deal with anti-Semitism in the party. The same day, Britain’s Chief Rabbi Ephraim Mirvis warned of “a new poison—sanctioned from the very top” taking root in the Labour party.  Although Corbyn apologised a week later, it was too little too late. The damage had been done and Corbyn had been exposed as a remorseless enabler of anti-Semitism. Corbyn’s image as a leader relied on his claims of moral authority. As a result, the incident severely undermined his election campaign. Corbyn and the Labour party worked tirelessly to demonstrate that Johnson was a morally bankrupt liar and cynic. But the anti-Semitism episode indicated a moral vulnerability within the Labour leadership as well, blunting many of his pre-election attacks on the Conservatives. This was a major blow to his election prospects and cost Labour dearly at the ballot box. 
The 2019 Election was the Brexit election. The implosion of the Labour vote was a direct result of its incoherent messaging n Brexit and Jeremy Corbyn's reluctance to commit to becoming a Remain Party. While accusations levelled against Corbyn over anti-Semitism within the Labour Party were damaging, they had almost no impact on the Labour vote share. Voters were happy to vote for the Conservative Party despite credible allegations of Islamophobia. This indicates that racism within the party was not a deciding factor in voters' decisions to back a party on election day.
[P1] Corbyn failed to get a handle on anti-Semitism in the Labour Party. [P2] This was a significant factor in its defeat in the 2019 UK Election.
[Rejecting P2] Voters were happy to back the Conservatives despite credible allegations of Islamophobia within the party. This indicates that racism and ethnic rifts in the party were not a deciding factor for voters.