Hitler was appointed to the position of Chancellor by President Paul von Hindenburg on January 30, 1933. Hitler benefited from the struggle between the Social Democrats and the Communists. The Communists refused to cooperate with the Social Democrats even though the two parties together outnumbered the Nazis in the Reichstag, even after the 1932 elections. German Communists were blinded by the hatred of socialists and by the belief that Hitler’s fascist ideas represented the last throes of capitalism and a communist revolution would soon follow. Social Democratic leaders pleaded with the Communists for a temporary alliance to stop Hitler, at one point even posing their pleas at the Soviet Embassy, but there was no use. Hitler demanded to be named Chancellor as a condition of joining a coalition government in the Reichstag.
The Weimar Republic constitution provided for rule by decree--that is dictatorship--in the event of a national emergency. When the Reichstag was burned by a communist sympathizer hoping to bring down the government, Hitler used this to persuade President Hindenburg to evoke the Emergency Powers provision of the Constitution. On March 23, 1933 the Reichstag passed the Enabling Act which gave Hitler dictatorial powers for four years. It was the beginning of the end.
The answer to this is that Adolf Hitler was appointed to be the Chancellor of Germany. This happened in January of 1933. Having been appointed Chancellor, Hitler took more and more power for himself and for the Nazi Party, eventually creating a system in which he had essentially unlimited power in Germany.
Hitler was appointed because the leaders of the government wanted to coopt the Nazi movment. They believed that appointing Hitler as chancellor would make his followers happy. At the same time, they thought, they could control Hitler so as to make sure that he did what they wanted. In this way, they would have gotten the benefits of having Hitler in power without having to allow him to truly control things. They were, of course, terribly mistaken.