Because Sanders is too liberal...?
At one time, Obama may have had more in common with Sanders, but today Obama is inside the tent, and Hillary is inside the same tent, and Bernie is the guy outside relieving himself.
Or look at it this way: Obama's made it necessary for Hillary to move further left to where his own position is. He makes her look good to the base and she makes him look good, and he has some control over how his people think about her.
Bernie, further left still, would make Obama look depressingly moderate to today's progressives. And he would be much harder to influence.