Labour and the Tories quite like things as they are; they get to run the show without having to work for the support of more than a small slice of the electorate and, at worst, they just need to switch chairs every few years. Anything other than FPTP would be political suicide for either of them.

Same for the Canadian Liberal and Conservative parties, as well the Democrats and Republicans in the US.

That said, I used to think more highly of PR until the right wing swept through Europe, but to be honest that has a lot to do with the left-wing parties ceding economic populism to the right because, frankly, they like being invited to posh parties and hobnobbing with the rich, as opposed to pounding the pavement trying to get working class votes. Plus, identity politics was kind of a feedback loop for them: they could look progressive while still staying on the side of the rich.