Communism is unpopular

The purpose of democracy is to constrain the size of the state. If there is no democracy at all and a government or leader is able to have absolute unchecked power then there is no limit to the tyranny which might result. Democracy constrains government. With the first past the post system of elections there is generally the emergence of a two-party system as a result of Duverger's law. What this means is that the mechanism which acts to reduce the size of the state is diminished. Democracy relies on choice and if choice is diminished in this way then democracy ceases to act to reduce the size of the state and two statist parties can emerge and control parliament. Since there is less democratic pressure on these parties both will be pro-government and left wing. Fptp encourages the emergence of two centre-left parties. Pr would enable voters to reject both of the centre-left parties in favour of a non-aggressive low taxation party. If the opposite of socialist is right-wing then pr enables right-wing parties to get into government. Fptp excludes (liberal and) right-wing parties from gaining access to the government.