lemongrove In my view, there are a number of reasons why Labour lost this election:
Corbyn put forward a radical set of policies from the outset, and from thereon he was characterised by almost every part of the mainstream media as unpatriotic, stupid, dishonest, hypocritical, anti-Semitic, and anti-business. Every day brought new accusations and a deluge of ridicule.
The Labour Party, to a greater extent than other parties, was split between remainers and leavers and Corbyn's honesty in refusing to align himself with either side, instead saying he would try and get a better deal which would be put to a further referendum for the people to make their decision without his championing of either faction, pleased neither leavers nor remainers.
Many people profess to be uninterested in politics and, in fact, characterise most politicians as untrustworthy, greedy and useless. They like simple answers, and welcome supposed simple solutions. Johnson's sloganizing (and refusing in many instances to face robust questioning - the media, in the main, facilitating that strategy) appealed to those who just wanted Brexit over and done with.
Historically, the mainstream media has always attacked Labour, whomever its leader may be. They did it with Foot, Kinnock, Brown and Miliband - but Corbyn in particular espouses policies which the rich and powerful would not stand for - eg stronger regulations relating to finance, employment rights, health and safety, environmental protection, etc, etc. Blair was the only Labour leader to be given an easy ride. His willingness to cosy up to Murdoch paid real dividends to his leadership and the party in the short term. But reducing regulation of the finance sector and gambling industry, and the invasion of Iraq resulted in long term and serious damage - international turmoil and the economic crash of 2008 - the fall out from which continues to this day.