I saw a good point recently that due to the current FPTP system and make-up of parties, that left-leaning ones are always at a disadvantage.
Basically people on the 'left' have the option of Labour, Lib Dem, Green and SNP. That's 4 different parties where votes for similar areas of the political spectrum will be spread thin. Put this against the Tories who have had very few cast-offs in recent years (UKIP, Reform). It means their votes are all concentrated for them whereas on the left it's diluted.