I've listened to the interview from Tom King on it. It's very complicated.
He wanted to do it as a comic but nobody bit, so then he moved into writing prose and switched it into a novel which he'd partly written. Then Vertigo became interested and it went back to being a comic, it was meant as an ongoing with a finite end at some point, they decided to do it in volumes with 12 issues being the first one.
He and Gerards are now making bigger waves on Batman so I think the idea is they can bring across a bigger audience after people pick up the trades when it returns.
I agree with Roberts that Vertigo is still very strong creatively but from an organisational aspect they are a mess. They have canceled so many books abruptly (and I know economically the sales are pathetic) you begin to lose faith. Unfollow was a really great book and had rave reviews but sold like shit. Their internally competing lines mean they give them no publicity even in their own house ads (I picked up the terrible selling Sandman at issue 8 after they did a publicity blitz with quotes from Alan Moore and others on how great it was).
It seems like they just see it as an IP farm, we may one day see the proper Unfollow story but on a TV channel like iZombie, which has lasted way longer than the comic.