Simple explanation: Hubris. Obama thinks he’s the smartest guy in the room at all times, so why should he call Carter?
“The way I approach problems is to say, ‘What’s the optimal solution?’ and then work backwards. I like to start with the idea—what’s true to me? Or what’s the best approximation of true?” ~Senator Barack Obama, 2005
So apparently, Obama is working backwards from the optimal solution and he hasn’t worked backward far enough to call Jimmy Carter.