If I recall correctly, of the titles of the three books (FotR, TTT, RotK), Tolkien liked "Return of the King" the least because it said exactly what happened in the book (well, not wholly, but you get the picture). So would not "The Downfall of the Lord of the Rings and the Return of the King" be even worse? I don't know about you, but I don't want to pick up a 1,000-page book and already know the result without having read the first word.
Besides, that long a title would be extremely wordy and irritating. Can you imagine: DotLotRatRotK. Yikes! It's a very cool title for
Frodo's book, but in fiction the title should be interesting, relatively short, and more or less to the point.
It is interesting, though. Most books take their title from the protaganist (a book called Frodo? Eh, not as interesting), but LotR is named after the antaganist - that is, Sauron, Lord of the Rings. Yet it is fitting - it is an appropriately shortened form of the title of Frodo's book, and it takes in itself the object around which the whole plot revolves, the Ring. It catches the eye, as well: I remember thinking when I first heard the title, "Rings? What's so important about rings? What kind of rings?" (Maybe I think too much.
)
LotR has the title it has because it fits.