It's all about the back story. This year will go down in Oscar history as the year of the protest over diversity. The idea of making Mexico's Inarritu only the third director to win back-to-back Oscars can be seen as proving that the Oscars are diverse and, irony of ironies, they don't have to honor anyone from the complaining group to do it.
Even without that particular back story, Inarritu would be the clear favorite, a director whose projects over the last decade have been part of Oscar history. None of his competition can match that.
McCarthy is an actor first, a writer second and a director third. His previous films as a director (The Station Agent, The Visitor, Win Win) were all actors' showcases as is Spotlight. He will win for his original screenplay and his film has a strong possibility of winning Best Picture. Oscar voters will likely see that as enough for him.
Abrahamson does a terrific job of directing two disparate halves of the same film, but he's not particularly well known and his film is not considered a front-runner for Best Picture. His nomination falls under the category of "welcome to the club".
McKay has a terrible track record as a director, despite which he is a strong contender for his adapted screenplay. He's lucky to be nominated for his direction.
Miller is a one-trick pony. He's the Sylvester Stallone of directors, beloved by many, but perhaps unlike Stallone, not likely to win.
“Life is a shipwreck, but we must not forget to sing in the lifeboats.” - Voltaire