the lord of the rings - Why was Tom Bombadil so unconcerned about the fate of Middle-earth?

I know he saves the hobbits from the barrow-wight and gives them weapons which affect the Nazgûl, Sauron's greatest servants. But I've always wondered why a high power, which Tom Bombadil obviously is, doesn't play a more significant part against Sauron. Gandalf says at the Council of Elrond that Tom wouldn't understand the struggle between Light and Darkness.

Asked if Bombadil might take the Ring for safekeeping, Gandalf replies:

"No," said Gandalf, "not willingly. He might do so if all the free folk of the world begged him, but he would not understand the need. And if he were given the Ring, he would soon forget it, or most likely throw it away. Such things have no hold on his mind. He would be a most unsafe guardian; and that alone is answer enough."

_{The Fellowship of the Ring, Book II, Chapter 2: "The Council of Elrond".}

What confuses me is that if Sauron regains the One Ring, Tom will also fall like the rest of Middle-earth.

Is there any other info in the texts (aside from fact that Gandalf says that he basically doesn't care what happens) about why he didn't play a more prominent role?

Answer

Tolkien addressed this in Letter 144:

I might put it this way. The story is cast in terms of a good side, and a bad side, beauty against ruthless ugliness, tyranny against kingship, moderated freedom with consent against compulsion that has long lost any object save mere power, and so on; but both sides in some degree, conservative or destructive, want a measure of control. but if you have, as it were taken 'a vow of poverty', renounced control, and take your delight in things for themselves without reference to yourself, watching, observing, and to some extent knowing, then the question of the rights and wrongs of power and control might become utterly meaningless to you, and the means of power quite valueless. It is a natural pacifist view, which always arises in the mind when there is a war.

^{_{The Letters of J.R.R. Tolkien 144: To Naomi Mitchison. April 1954}}

Basically, Bombadil isn't interested in the threat of Sauron because he is entirely beyond caring about "good" or "evil". In fact, Tom Bombadil doesn't really care about anything beyond preserving the peace within his own borders; even that he only accomplishes with song and force of personality.

Bombadil is ultimately a complete pacifist, and has no use for violence or struggle against anyone, even "evil".