The story is in the data
The data shows that Brexit was a major factor in voter's decision at the ballot box.
The structural shifts brought about by Brexit dismantled Labour's base.
Labour has only been a potent force in UK politics because of the alignment of the working-class vote with university-educated middle-class voters.
The Labour Party’s strength stemmed from the alignment of the working-class with university-educated middle-class voters. Both groups came under the Labour banner because their economic interests aligned. They both wanted higher wages, stronger trade unions, quality and accessible public education and a world-class health system that was free at the point of use.  However, Brexit created a seismic jolt in British politics. The jolt pushed the economic interests of these two key demographics out of alignment. Unless it radically changed the core focus of the party, Labour was always doomed to lose the 2019 election. In the run-up to the 2016 referendum, there were already warning signs. The economic interests of the working-class had moved to immigration, while the middle-class had maintained their focus on wages, free trade and the free movement of people. The 2019 election was the result of these two core groups of Labour supporters falling out of alignment. Working-class voters flocked to the Conservatives over concerns that their economic interests were not being heard in Jeremy Corbyn’s Labour Party.
This wasn’t the result of Brexit; it was the result of Corbyn’s leadership. Labour was not a hard Remain Party like the Liberal Democrats. They campaigned for a second referendum, not to rescind Article 50. Working-class voters did not turn away from Corbyn because of Brexit, they turned against him because they were unconvinced by his radical left manifesto that touted the evils of capitalism. In one poll, voters said the second-biggest concern with Jeremy Corbyn as Prime Minister was that he would spend too much and put the country into more debt. This has nothing to do with Brexit or immigration, just Jeremy Corbyn’s reckless fiscal policy.
[P1] Labour relies on the support of the working-class and university-educated middle-class. [P2] The two demographics have traditionally shared economic interests. [P2] Brexit pushed the two group's economic interests out of alignment. [P3] This dismantled Labour's base and caused their election defeat.
Enter the technical rejections of the premises here ...