Because Chandler's character was the lead and Ben's wasn't, it has nothing to with who was better or more important to the story, often the supporting actor will have the more memorable scenes.
I agree with you though Mendelsohn is clearly the better actor here.
really? I thought Mendelsohn had enough scenes (as much as anybody else) and importance to be seen as the lead here. it is only down to award politics. not that it really matters, but its sticks out like a sore thumb