Truman fired macarthur: for incompetence. because of party politics. to preserve civilian control of the military. for reasons that are still unknown. because he never liked him personally.

To preserve civilian control of the military

Dwight Eisenhower had been a military general himself (during World War II).  But now he was in the position of civilian commander in chief as President of the United States.  General MacArthur wanted to bomb the People's Republic of China and use Nationalist Chinese forces from Taiwan also in an attack on communist China.  President Eisenhower would not grant these requests to escalate the war, and MacArthur made his opposition to Truman's policies a public argument.  Eisenhower felt he had to relieve MacArthur of his command and maintain respect for civilian control of the military, with the President as Commander-in-Chief.