Because Labour and the Conservatives benefit from FPTP. Both parties have won large majorities on minority vote shares, and both know that PR would make it much harder for them to govern alone.
Labour flirted with electoral reform under Tony Blair, but dropped it once they won a landslide in 1997. Keir Starmer has ruled out electoral reform, despite pressure from the Lib Dems and the Greens.
The only way reform will happen is if:
The 2024 election produced the most disproportionate result in UK history: Labour 63% seats on 33.7% votes, Reform 0.8% seats on 14.3% votes. FPTP creates wrong-winner elections (1951, Feb 1974), safe seats where 90% of votes are wasted, and governments with absolute power on minority support. Every major democracy except UK, USA, and Canada uses proportional representation, with better outcomes: higher turnout, more women MPs, less polarisation. The case for PR is overwhelming but Labour and Tories benefit from FPTP and will never reform it unless forced by electoral collapse or coalition demands. If we don't adopt PR now, the UK will continue its democratic decline: falling turnout, rising extremism, and governments that represent 1/3 of voters ruling 100% of the country. First past the post is not fit for purpose. It is a relic of the 19th century that produces 21st-century injustice. We need proportional representation, and we need it now. The 2024 election was the final warning. If we ignore it, we will sleepwalk into democratic collapse.