Labour’s campaign. I’ve heard several people who want to put the blame for the loss entirely on Brexit point out that it couldn’t have been the fault of the campaign or of Corbyn, because they were essentially unchanged from 2017, when Labour did far better than expected, rather than worse. That, however, is part of the problem - it was something of a repeat performance, and couldn’t hope to have the revelatory freshness of two years ago: even the campaign slogan was recycled. Also, although Corbyn did numerous outdoor events that were well attended (including a very successful rally here in Bristol a few days ago), the fact that it was happening in a cold, wet season, which gets dark at 4.30, meant that the kind of stump campaigning where he excels was necessarily limited. In studio interviews he was far less effective: there were no major gaffes, but he often came across as querulous; and while he was competent enough in the debates, he failed to deliver any killer blow. I know that politicians these days are schooled to stick to a few key messages (e.g. the NHS), but I wish he had done more to highlight the obvious weaknesses of the Tories (their abject economic failure, the many lies and broken promises of their leader, etc.).
Brexit. This was almost certainly the most important factor. Both major parties were split by Brexit, but there was an asymmetry that was fatal for Labour. Johnson could afford to be ruthless with his remain wing (sacking 22 MPs, for example) and still be sure that a) he’d keep the vast majority of Tory voters, and b) maybe attract some Leave-supporting Labour voters into the bargain. Corbyn’s voters were split far more evenly, and he had to try to please both, with the predictable result. Nor could he count on picking up disaffected Tory Remainers in the way that Johnson could absorb Labour leavers, since they had an alternative home in the form of the LibDems. Finally, the Brexit party was able to mop up a small but, in many constituencies, decisive number of Labour leavers who couldn’t bring themselves to vote Tory. The idea, being floated by several FB friends, that Labour would have walked the election had it been led by a centrist Remainer, just doesn’t stack up: anyone would have been caught on this particular forked stick, and the only comforts are that a) by the next election Brexit will, presumably, not be an issue and b) many of those Tory majorities in Labour heartlands are very small and eminently win-backable. (I do think, though, that Corbyn would have been far better advised to promise a quick referendum on Johnson’s deal vs. Remain, rather than offering to negotiate a Leave deal of his own, which he would then be neutral on. The effect of that was beyond messy.)
Mendacity and the media. I’m not going to complain about media bias, which in the UK is just a fact of political life - most obviously, but by no means exclusively, in the printed press. However, the media as a whole, and broadcasters in particular, appear to have been badly wrongfooted (why I don’t know, since it was entirely predictable) by the Tory policy of mendacity on a Trump/Bannon scale. In interview after interview, Johnson and others were allowed to lie unchallenged, as well as being able to renege on agreements (e.g. the Andrew Neil interview) without consequence. BBC reporting was particularly supine, Channel 4 rather more robust. When Johnson denied that there would be checks for goods entering N. Ireland, for example, contradicting the deal that he had himself reached with the EU, the BBC website tucked the fact that it was untrue into the tenth paragraph of its report, headlining instead with Johnson’s words. Another example, that in some ways sums up the rest, is the reporting in recent days of the result of a fact-checking investigation into campaign ads, which showed that 88% of Tory ads had contained untrue statements, the figure for Labour being 0%. The BBC website apparently thought that balanced reporting of this finding required them to state that there had been mendacity “across the political spectrum.” That is not what good journalism looks like, and in times like these we need good journalism.
I’m sure there are other important factors, but these three seem to me the most decisive, with the second probably preeminent. If I have time, I may do a second post to pair with this one, looking to the future - but it will be shorter.