There's also the fact that Syngman Rhee was opposed to the monarchy returning as he wanted to maintain his own grip on power, going as far as to seize all their properties and such.
You would at least need to replace Rhee with someone else, but I do not know of any potential Korean leader who was keen to restore the monarchy. Even if there was one, they'd be one of the least likely people to be propped up by the Americans.