Polls. As soon as his popularity starts to drop, they begin to see more people (readers) interested in reading about his failure than his worship. It's all about selling papers.
Obama had the makings of a star, and a star he became. Celebrity power sells, and the media was happy to sell it with him. Now his administration has the stink of failure escaping from the seems. Disrespect abroad, economic woes at home, arrogance from the White House, and sinking poll numbers. You can already feel the winds shifting.
Broken celebrities are worth far more to newspapers than intact ones.