Took's explanation is brief but on the money. Just an added note- there was some mention about Bilbo, Frodo and Sam being given the opportunity to sail into the West (Sam later on) because they both bore the One Ring for a time. But then in one of the Appendices of the Return of the King there is mention of Legolas and Gimli getting on a boat and sailing into the West. Tolkien wrote a vast and complicated epic and there were bound to be some inconsistencies and loose ends here or there. Too bad the man didn't have a laptop. It's staggering to think what he might have accomplished.