Labour failed to lay the necessary groundwork for many of the policies contained in its 2019 manifesto.
The Labour manifesto's tack to the left and embrace of radical policy ideas alienated swing voters and independents.
Labour’s 2017 manifesto drew support for the party and helped it put in a strong performance at the polls. In 2019, it doubled down on the radical policies that had helped it in 2017, to disastrous effect.
The policy proposals laid out in the 2019 manifesto read like a wish list of the radical left. Labour promised to eradicate tuition fees, boost infrastructure spending, embark on a campaign of renationalization, inject more money into pensions, and give away free broadband across the country.  The pursuit of these policies was damning on all fronts. The people these policies would appeal to, those on the far left of the political spectrum, would already be voting for the Labour party. But the policies were too radical to pick up swing voters or independents and were simply repellent to Conservative voters. Therefore, the manifesto failed at its objective of attracting new voters to Labour. Even centrists that traditionally voted Labour would have found these policies difficult to stomach without a comprehensive plan for how a Labour government would pay for them. In publishing such a radical manifesto, Labour misread the situation and missed a golden opportunity to pick up Conservative votes. With former Remainers leaving the ranks of the Conservatives, had the party taken a more centrist approach, Labour could have drawn in voters let down by the Conservative’s Brexit policy.  Rather than an asset to attract new voters, the radical manifesto was a lead weight on a sinking Labour ship.
Labour’s individual policies, particularly the left-leaning ones, were highly popular. Support for public ownership of the UK’s water, gas and railway companies sat at around 70-80% nationwide. More than 60% of the electorate also supported increasing taxes on high earner’s.  The fact that these policies on their own polled well would indicate that it wasn't the content of the manifesto that was unpalatable to voters, but perhaps the messenger itself. Corbyn was a highly divisive figure and lacked the charisma and charm to win the game of personality politics that a strong election performance requires.
[P1] Labour's manifesto was too radical to attract new voters. [P2] To win more seats, Labour would need to win over some new voters. [P3] Therefore, the manifesto prevented Labour from performing well in the 2019 General Election.
[Rejecting P1] Individually, the policies polled well among the electorate. Therefore, it wasn't the manifesto's content that was off-putting to voters.