Preferential Voting

I see that the Labour Party used Preferential Voting in their quest to select a New Party Leader.
Preferential Voting method a.ka. as the instant-runoff voting (IRV), or the "alternative vote"  which uses voters' preferences to simulate an elimination runoff election without multiple voting events.
As the votes are tallied, the option with the fewest first-choice votes is eliminated. In successive rounds of counting, the next preferred choice still available from each eliminated ballot is transferred to candidates not yet eliminated.
The least preferred option is eliminated in each round of counting until there is a majority winner, with all ballots being considered in every round of counting.
Why then do they then insist upon a First Past The Post voting System in a General Election?