Because he can control him. This from th Washington Times. An extremely interesting article that basically says what I've believed all along.
Mr. Obama needs a successor whom he can control to ensure that the “fundamental transformation of the nation” continues. He cannot control either of the Clintons. In a revealing “tell” this week, he said, “In 18 months, I’m turning over the keys. I want to make sure I’m turning over the keys to somebody who is serious about the serious problems that the country faces and the world faces.”
Read the remainder of the story @ MONICA CROWLEY Obama a threat to Hillary Clinton - Washington Times
Biden Reportedly Exploring Presidential Bid – Of course he is! @ http://www.newsweek.com/biden-reportedly-explores-presidential-bid-359069
He's waiting while Billary continues to shoot herself in the foot.