The totalitarian states usually don't allow any opposition parties to exist. If some people try to form a party, they usually end up in prison, are banished from the nation, or even murdered.
The reason why the totalitarian rulers prefer a one party system is in order to have all the power in the country, without having opposition that will constantly go public about the terrors and injustices that are happening. If there are more parties, than the people will start to support them, especially cause rarely who likes to live under a totalitarian ruler, thus the totalitarian ruler will face a situation where he/she will lose its power.
