There are two major reasons for this.
First, MacArthur was a major hero in World War II. He was the person most closely associated with American victory in the Pacific. That, alone, would have been reason enough for people to welcome him home as a hero only a few years later.
Second, MacArthur got in trouble for being too tough on communism. At a time when the US was very worried about communist expansion, it was no great sin in most people’s eyes to advocate really taking the war to the communists.
Thus, MacArthur was welcomed as a hero both for past services and because of the fear of communism that was prevalent at the time.