A one party government does not have any checks and balances. These checks and balances prevent any single group of people from obtaining complete power. While one party may have power to enforce the law, another has power to enact laws, and a third party has the ability to make certain the laws are just. In this way, power is evenly distributed.
If there are going to be multiple parties running the government then that could obviously lead to more arguments and debates and therefore contribute to wasting more time than we can afford to. It may be better to have a one party rule that way people will be more united.