Corbyn’s delay in responding to the Salisbury attack, in which a former Russian military officer and his daughter were poisoned, and his refusal to denounce Russia as the perpetrator was just one incident in a series of gaffs that demonstrated his weakness on issues of national security. 
Corbyn has a checkered past with matters of national security, particularly those related to the Eastern bloc. In the 1970s, communist East Germany invited him behind the curtain on a tour due to his left political leanings and communist sympathies. His past ties to communist regimes mean Corbyn would never be trusted with sensitive information if he applied for a government job. He wouldn’t pass the necessary background checks for high-level clearance in MI5 or MI6. Therefore, how can anybody be expected to vote for him as prime minister? Among his many unpopular policies on national security was the pledge not to renew Britain’s Trident nuclear deterrent program. Many voters saw this as a direct threat to UK national security and determined that he could not be trusted in a prime ministerial capacity to keep the country safe. This ultimately caused him to suffer at the ballot box.
The alternative was far worse than Jeremy Corbyn. Boris Johnson was not exactly a safe pair of hands protecting the UK’s domestic security. Boris Johnson’s government refused to release the Parliamentary Intelligence and Security Committee Report which analysed Russian meddling in the UK Brexit referendum. Under the Conservative government, army personnel has fallen from 102,000 to less than 74,000. The Royal Air Force (RAF) and the Navy have undergone similar cuts. Additionally, military staff have had to settle with seven years of pay increases below the national inflation rate.  There is also evidence to suggest that voters see Labour as stronger than the Conservatives on national security matters. Labour doubled its vote share in 2017 in places like Aldershot, which have a large military presence, even with Jeremy Corbyn as the party leader. This exposes the invalidity of arguments which suggest Labour lost the election due to the public perception that Jeremy Corbyn couldn’t be trusted on national security.
[P1] Jeremy Corbyn cannot be trusted with matters of national security. [P2] Voters knew this and it was a major factor in their decision not to vote for the Labour Party.
Rejecting the premises
[Rejecting P1] Corbyn was stronger than Boris Johnson on national security matters. [Rejecting P2] National security was not a major factor in voters' decisions.