Why was Abraham Lincoln chosen to be a vampire hunter?

The movie is based on the novel of the same name.

From an interview with the author Seth Grahame-Smith (2010):

When you got the idea for this book, were you thinking, “I sure do love Abe Lincoln, but I wish his story had more vampires?” Or was it, “I’m sick and tired of all these vampire novels without any historical context?”

• To be honest, I don’t know which came first in that chicken or the egg scenario. I’ve been a lifelong horror fan, but at the same time, I would say 90 percent of my reading is biographies and nonfiction history. I read a lot by Doris Kearns Goodwin and David McCullough and Walter Isaacson. I’ve always enjoyed reading history, particularly presidential biographies.
  
  I guess where it really came from was, I like to wander around bookstores and see what people are reading. Last year, during the Lincoln bicentennial—and this was when 'Twilight' was really blowing up—it seemed to me like every book on the front shelves was either a Lincoln biography or a vampire novel. So it was like an instantaneous click in my brain.

You thought, “If I can find a way to combine these two genres, I will rule the world?!”

• (Laughs.) Something like that. It was a weird chocolate-meets-peanut-butter moment for me. It was a chance to take my two favorite tastes and put them together.