Actually, them haters forgot that this 16k is under the circumstance that all the stores selling Horizon were practically emptied of every single copy of the series. (it's quite a hyperbole, but
)
Imagine what would Horizon's sales would be if it actually released a higher number of copies. Those copies would still be flying out of the shelves, I'm sure.
One big reason why Horizon is selling this much is the seiyuus, the staff and practically the original creator put lots of love in this series.
I think this is the first time I've heard of a series (an adaptation, even) that even the seiyuus fell in love with the original work. You just have to give Kawakami-sensei credit for that.