Book Series

Question: Why are book series so popular? Is it the suspense you get when you reach the last page, the last word, and know you simply have to read the next installment? My friend Karen Vogel, an excellent writer, writes Amish Fiction, which seems to be in vogue, and all her stories save two books thus far are in installments, about thirty pages each or so. Believe me, they are worth the purchase, for she fills each page and paragraph with truths, lessons, and wit which we can all relate to in one way or another. I've never read Beverly Lewis or Suzanne Woods Fisher, but Karen ties in all these wonderful concepts and parallels them with classic literature, like Pride and Prejudice, The Secret Garden, and Mansfield Park, to name a few.
But back to the thing about series.... I have nothing against them, I'm just curious to see what people say. Is it the writers? I wonder if it is easier to write in segments because then you can focus on certain things and hone your story's focus with each tidbit. Consequently, I think that tactic also makes the series longer if they're, say, condensed into a single volume, but I don't mind lengthy books. In fact I prefer them. Still haven't tackled either volume of Les Miserables but I'm gearing up to it. The musical is wonderful.
What do you think? Are you as a reader or writer attracted to series? Why or why not?

-Grace