It is extremely hard to prove which factors mattered to voters and the possible explanations will be polluted by groups pursuing their specific agendas
Voters make broad judgements about parties and their leaders during elections. And voters are not uniform. For example, many vote out of long term loyalty to a party with little thought or analysis; others switch voted depending on local issues or the personality and stance of their local MP; some consider which party leader would make the best Prime Minister on a variety of reasons; a few (and it probably is a small minority) look at the policy proposals as a whole and weigh up which they prefer.
Given this, producing a single explanation is impossible and we should be suspicious of any argument that claims a single factor in policy produced the result.
Another reason to be suspicious of single factor explanation based on policy differences is that many voters also consider competence an issue. If a party has a bunch of great policies that are themselves popular but people judge the party or its leader incapable of delivering those policies, the policies will be irrelevant. Activists inside parties rarely seem to understand this basic point.
Consider how various factors might have influenced the relative perceptions of leader and party competence in the 2019 election.
While any reasonable analysis of Johnson would not highlight his competence or honesty, he was clearly in control of the party (despite opposition which he mostly disposed of) and created a manifesto with a clear and simple message ("get brexit done"). Many voters disagreed with that policy, but at least it was clear.
Corbyn was not clearly in charge of his party. His MPs were uncomfortable with his leadership. He had failed to crush antisemitism (many supporters claims this was a media plot and he is clearly not an anti-semite which might be true but isn't the point). The problem wasn't anti-semitism, it was his incompetent management of it in the party. Then the policies and manifesto. There were something like 50 key policies many designed to appeal to particular groups. But they were sometimes poorly thought through, frequently contradictory and lacked any sense of focus or priority. Swing voters whose concern was competence looked at the manifesto and saw no hint of priorities and no simple message they could grasp. Trying to appeal to many individual groups of activists sent a signal that the party was poor at prioritising or making difficult choices, not a good quality in a possible government.
Whether the lists of policies were good or bad is irrelevant to many voters. They want to see signs that you are competent enough to run a government when hard choices and clear priorities cannot be avoided. The Labour party failed to send those signals.
Was Brexit a big issue? Pro-brexit Labour people think it was key, arguing that Corbyn should have taken a clear stance that didn't risk undermining the referendum. But the evidence that voters abandoned Labour because of the specific Corbyn stance is weak: both pro and anti-brexit voters left Labour. But this misses the point for the voters concerned with competence. Ambiguity was the problem. A clear stance either way might have been better.
But the overall point is that a focus on policies or single factors is unlikely to provide a clear explanation for why Labour lost. Voters are complicated and are often influenced by a general impression of whether a party is competent rather than a specific policy stance or factor.
