MacArthur was a person of remarkable personal courage (SEVEN Silver Stars in WW I, at least a couple that could easily have gotten him the Medal instead). He was also an egotist of the first order of magnitude, a person with absolutely no concern or compassion for others, a person who saw and exercised duty and honor in a way that spit on his beloved corps and the nation which it serves.
There are remarkably few famous Americans who I can think of who would have been a worse president (Arron Burr comes to mind). The thought of MacArthur with no one holding his leash is literally terrifying.
So, you must have enjoyed his ... downfall in my Prodigal Songs TL
In all seriousness, Mac had some good traits; he was a remarkably intelligent man, he was in favor of integrating the army, and so forth. But, as others have said, he was an egocentric who had little but disdain for civilian authority, and saw him as a Great Man (the later isn't neccesarily terrible; many in the course of history have seen themselves in the same light; some of them were even right)
Also, this was an age when the Nuclear Bomb was viewed as a conventional (albeit, drastic) weapon of war. Even Eisenhower possessed such an attitude, until the end of his first term/beginning of his second, where his thinking made a dramatic turn. Mac isn't the type of man who would be able to make such a turn of thought (he was someone who often refused to admit when he had been wrong, even when it was obvious)
Also, the use of Nuclear Weapons as a conventional weapon during this period would have set the dangerous precedent of their use during 'conventional' war. Also, it would have seriously hurt, if not destroyed, their prestige as "war enders". This would have been very bad once other natiosn begin to develope their own arsenals (and it would have become even more imperitive that every nation would want/need to develope such a stockpile in that environment.)