Tried to be everything to everyone
The manifesto tried to do too much, and in doing so, did nothing.
Labour failed to lay the necessary groundwork for many of the policies contained in its 2019 manifesto.
The former Conservative veteran campaign manager, Lynton Crosby, often reminds politicians that “you can’t fatten a pig on market day.” Selling policies takes time. It takes years of alerting the electorate to the nature of the problem, and clear messaging strategies to illuminate precisely how your party’s policy can fix the issue. These steps were absent for many of the policies in Labour’s 2019 manifesto.
For many of the policies in the manifesto, there was no preparation. Instead of spending years making the case for the policy and drumming up public interest, the manifesto dumped it on the electorate with no warning or preparation. Labour announced campaign promises such as the four-day working week and free broadband without effectively identifying the problem they sought to address. The electorate woke up one morning with these landmark policies included in Labour’s election pledges. Without adequate preparation, even policies that polled well among the electorate did little to improve Labour’s performance on election day. The free broadband policy pledge, for example, was sprung on an unprepared electorate late on a Thursday night, not even at the manifesto launch or major campaign event. It was never brought up at the Labour Party conference. There was a strong case to be made for providing free broadband. Private companies have not done a good job of modernising the country’s broadband network, and a stronger internet connection benefits everyone, from the lowest economic classes to the top evenly.  However, in the end, none of the groundwork was effectively laid to articulate this to the population. The announcement was rushed and poorly planned. As a result, the impetus of this policy, like many others, was lost on the electorate.
Enter the counter arguments here ...
[P1] For policies to resonate with the electorate, they need to clearly address an issue that the electorate has been made aware of. [P2] Labour did not effectively demonstrate how their policies would solve a clearly-defined problem. [P3] Therefore, Labour failed to adequately prepare for their manifesto policies. [P4] This cost them the 2019 election.
Enter the technical rejections of the premises here ...