Labour's vote share was comparable to previous elections
In many areas that Labour lost to the Conservatives, their vote tallies were similar to previous elections in which they had won the seats.
The Conservative Party ditched the unpopular policies of days gone by.
Under Boris Johnson, the Conservative party ditched the deeply unpopular austerity program of the Cameron and May years. This meant that they could pick up working-class votes who would never have voted for an austerity program, but wanted a Leave party in power. 
The Conservatives adeptly recognised that voters no longer care about austerity. They want a big state willing to spend on issues that matter like the NHS and the police, particularly the working class, whose support the Conservative's would need to wrestle control of the North from Labour. For the first time in more than a decade, their messaging steered clear of excoriating Labour for its reckless spending under Blair and Brown and pushing the national credit to the limit. This time, the Conservative messaging was more willing to talk about spending money than saving. Sajid Javid revealed that the government would increase spending by 4.1%, the largest expenditure increase in 15 years. There are also to be no spending cuts.  The Conservative’s victory in the 2019 general election stems from their exceptional ability to read the electorate and reinvent themselves as the party of post-austerity.
Enter the counter arguments here ...
[P1] The Conservatives needed to win working-class votes to make up a majority. [P2] The working class would never have voted for an austerity party. [P3] They ditched the austerity messaging. [P4] They won the working-class constituencies they needed for a majority. [P5] Therefore, the abandonment of austerity played a key role in the Conservatives' election victory.
Enter the technical rejections of the premises here ...