I would assume they're too afraid of sending our best and brightest young astronauts to be blown to tiny bits again and rather not lose Discovery, Atlantis, and Endeavor. They need them to put in museums, and if you count Enterprise (I'm still pissed it never got to fly in space) they can say only a 3rd of them blew up.
Also, since Challenger blew up during launch and Columbia blew up while attempting to land, NASA still hasn't lost a man in space, and they want to keep it that way. They recently installed the new solar panel on the ISS, and I guess that means they don't need the shuttle cargo bay for a while.
They can still use the Russian rockets to move people to and from the station. NASA doesn't get a great budget, so maybe now they can launch more probes and focus on the new capsules that are replacing the shuttles.