Why did all the Ringbearers leave Middle-earth in the end?
At the end of The Return of the King, Frodo, Bilbo, Gandalf, Elrond, and Galadriel left Middle-earth on a ship, along with other elves. I named those five because they were all ringbearers and it seems to be a common trait. Frodo and Bilbo bore the One Ring and the three others bore the three elven Rings.
Why did they have to do that? Why didn't they stay and have a peaceful life now that the Dark Lord was gone?
@Kevin He left in the book, but there's no evidence in the movie version to suggest that he eventually left, and since in the movie the ship that Frodo and Bilbo took was the *last* ship to leave for the Undying Lands, it's hard to see how he could have done so.
@MikeScott The movie isn't canon where it conflicts with the book (and arguably at all), and if you want to argue that, Sam wouldn't have needed to go as he hadn't put the Ring on, in the movie.
@MikeScott The question doesn't say that it's specifically about the movie canon. If not specified, people usually default to the books. And quite reasonably so, since they're a *lot* more detailed in information (especially counting in the other canon books), have been around for a lot longer, and are also the original version of the story.
@Kevin Ah, my bad. Admittedly it has been a very long time since I read the books.
Elrond (by his choice) and Galadriel (by her blood) were Elvenkind and destined to grow weary of Middle-Earth. They long to take the Straight Road1 to the Blessed Realm as time goes on; they do not pass their days forever in Middle-Earth.
The Hobbits Frodo and Bilbo (and later Samwise), although of Mortal kind, by virtue of having borne the One Ring also bore a weariness in their bones. But in Middle-Earth they could find no relief from this. Although the Gift of Men (halflings being of Mankind) might bring them relief upon their death, they were granted a boon to travel to the West and pass their long days in peace until the Gift was granted them.2
1Since the Bending of the World, the only way to travel to Aman was by Elven ships following the Straight Road.
Istar is wizard, istari are wizards of ME. What do you mean by - "Gandalf was Istari and did not belong to Middle-Earth anyhow."
@Secko Istari are actaully Maiar, and come to Middle Earth from the outside. Unlike a lot of fantasy literature, they're *not* just individual men (or some other race) who just happen to possess ability to use magic, but rather could be compared to demigods or avatars of some sort.
I have clarified my statement and provided what I hope are helpful references.