In general when a new ideology appears there will be very few POPs who have that ideology. In general it will take many years for for a socialist party to become a serious contender in a countries politics. The parties are only in because you may have a highly unusual game which causes a communist revolution in 1850's. So rather than endeding up with a liberal government you will get a communist one. Another reason was to give you the player more political options, sooner. However in the end it is up to you, if you want to mod the parties file, please feel free, bacause the most important thing is that you should enjoy Revolutions.