An additional consideration is that a lot of books and other properties are getting converted into series television, and that length is just about perfect for converting a book into television without dropping significant portions of the content.

Examples might be Stephen King's The Outsider or Lisey's Story.

That said, unless the series is self-contained, it's rare that I'll start watching a new series that doesn't have at least a couple of seasons of content. Hate watching something new only to have the studio cancel it and leaving the story unfinished and incomplete.