Ignoring the will of the people
Labour's decision to position itself as a Remain party ignored the will of the people as articulated in the 2016 referendum.
The data shows that Brexit was a major factor in voter's decision at the ballot box.
Artificial intelligence analysed millions of social media posts surrounding the 2019 General Election. They found that the Labour Party’s Brexit policy was a key factor in the party’s loss.
As the campaign progressed and polling day approached, social-media posts became less focused on issues that Labour performed strongly on, like the NHS, and became more Brexit-focused. This is the clearest indication that it was the party's stance on Brexit, and that alone, that resulted in the devastating defeat at the ballot box. 
Social media conversations are by no means an entirely accurate reflection of the British electorate at large. Using social media posts as a data set would skew the data towards younger, more left-leaning voters. Because many older, more right-wing voters are not on social media, or do not engage with it as frequently, it is impossible to draw conclusions about voting patterns based on social media posts alone.
[P1] Social media discussions indicate the priorities and opinions of the population. [P2] During the election campaign, social media posts were highly focused on Brexit. [P3] Therefore, Brexit was the defining issue that the election was won and lost on.
[Rejecting P1] Social media discussions do not indicate the priorities and opinions of the entire population.