Why is the epilogue of "Eclipse," by Stephanie Meyer, written from Jacob's point of view? If the whole thing is written in Bella's point of view, then why is just the epilogue written from Jacob's view?
Speaking generally, an epilogue will contain information, story points, points of view, etc that were not part of the main story and are instead there to help wrap things up or set up another story.
First off, a quick note - I have never read "Eclipse", know nothing of Stephanie Meyer's work, and don't know who Bella and Jacob are.
That said, let's talk epilogues!
An epilogue is the continuation of a story, wrapping up some points, setting up the next is a series, etc. The key thing here though is that the material in the epilogue wasn't deemed to be part of the main story - it deals with the aftermath of the story.
Ok - so from what I gather from the question, the book "Eclipse" deals with a story - both the series of events and reactions to those events - from the point of view of Bella. She is the narrator of how the events unfold and is, in fact, who the story will centre on - how what happens affects her, how it makes her feel, how she observes others having their reactions and how she feels and reacts to them, etc. And so the story is not so much about what happens but more about how she feels about it.
But what to do with what happens after the story, those events? Should we observe the world from a 3rd-person point of view and see how everyone feels? Should we continue to follow Bella? Or should we keep with the 1st-person storytelling but switch to someone else's point of view? This is an important question the author faced, especially if Jacob is privy to story that Bella isn't but will help explain things, tie things up, or set up the next book.