Because it would suit their style, juxtaposing two contrasting things (classical literature and marketing-driven pop culture that has lots of sequels).
I get it. To take the joke even farther, you might imagine a headline from The Onion along the lines of "God working on sequel to The Bible". Not funny when I state it all dryly of course, but hopefully you can see the joke now?
Perhaps; authorship is disputed, and some claim the Book of Mormon is the sequel from the original author.
(Of course, its worth noting that that the problem goes back further than that and that "the Bible" is itself a combination of the Hebrew Scriptures and a -- of disputed provenance, like the others -- sequel to that work.)
