Couldn't be trusted on national security
Jeremy Corbyn repeatedly demonstrated that he could not be trusted to protect British national security interests.
Corbyn's support for the Irish Republican cause went down horrendously on doorsteps.
Many Labour canvassers said that Corbyn’s support for the Irish Republican movement often came up in conversations with voters. 
Jeremy Corbyn has a soft spot for Irish Republicans and the Irish Republican Army (IRA), a known terrorist group. He invited Linda Quigley and Gerry MacLochlainn, both IRA affiliates, to Parliament barely two weeks after the organisation had killed five people and nearly assassinated the prime minister in 1984.  In the 1990s, he also publicly supported an IRA bomb fixer who was on trial for producing bombs and supplying machine guns to the IRA from his London flat.  Most recently, he has refused to condemn IRA murders during a phone interview on 5 Live in 2015. Five times he was asked to do so. He eventually hung up the phone.  The Labour party can deny his support for the terrorist group all it likes, but the British public saw the evidence and concluded that Jeremy Corbyn was a clear supporter of the violent extremist group. This ended up harming the party’s 2019 election performance, as proven by the persistent raising of Corbyn’s IRA links on the doorsteps.
The charge of terrorist sympathizer has been levelled at Corbyn to cover up the government’s failings to prevent and curtail international terrorism. When Jeremy Corbyn opposed proposed airstrikes on Syria over concerns it could be used as a recruitment tool for international terrorist organisations, David Cameron called him a terrorist sympathizer. Again, when the Manchester Bombings took place, to avoid criticism levelled against his own party, Boris Johnson was quick to draw attention to Jeremy Corbyn’s history with the IRA. Johnson has, on several occasions, shared photographs of Corbyn with various Sinn Fein politicians from the 1980s and 1990s to illustrate his support for the Irish Republican cause. But these photographs illustrate nothing. Many Fenians met with international politicians, including Bill Clinton and Tony Blair. Politicians were constantly meeting with Republicans in an attempt to find a pragmatic solution to the troubles in Ireland through open dialogue. The existence of these photos does not prove that Corbyn supports the IRA or the Irish Republican cause.  Secondly, the UK Conservative Party is enacting a set of double standards when it accuses Corbyn of sympathizing with violent groups. Conservative foreign policy under David Cameron and Theresa May has maintained strong ties with the Saudi government, who stand accused of deliberately targeting civilians in their civil war against Yemen. In the 1980s, Conservative Prime Minister Margaret Thatcher supported violent militant groups across the globe, from Cambodia to Angola. Any charge of “terrorist sympathizer” against Jeremy Corbyn reeks of double standards from the Conservative Party. There is also limited evidence to suggest that the perceived links with the IRA played a major role in Corbyn's electoral defeat. 52 of the 54 seats the Conservatives took from Labour were in vote leave areas. Given Labour's emergence as a Remain party, this would indicate that Brexit played a far greater role in Labour's election defeat.
[P1] Corbyn's ties to the IRA frequently came up on the doorsteps in conversations with voters. [P2] Therefore, it can be concluded that his IRA links harmed Labour's election chances.
[P1] Corbyn's ties to the IRA were not real, but the result of a Conservative smear campaign and the pro-Conservative media. [P2] Brexit played a far greater role in Labour's electoral defeat.