Totalitarian governments are often controlled by a single political party because

Totalitarian government is when a single group completely takes over and makes the people subservient to the state (they make all of the rules) and if there was more than one group, opinions may differ and actions may be more diverse, and it wouldn't have total control over the decisions anymore. Ex: Hitler - changed laws to what he saw fit, changed culture, controlled how people were treated, threw Jew's in concentration camps because of a personal bias, etc. He had total control, thus totalitarian gov.