The FPTP voting system means everyone is held hostage voting for the party they hate the least. If you vote for a third party you just make it more likely the party you hate most will win because the vote is split.
Obviously things still wouldn’t be perfect with a proportional voting system but I think it would take some power away from capitalist oligarchs because we will be able to vote for a party we like without just making the fascists more likely to win. The issue is the only two parties you can realistically vote for are highly incentivised to not change the voting system because they will lose a lot of the power they have.