SpaceX spent less than a billion dollars developing the Falcon Heavy. NASA has been spending a couple billion a year on the SLS, a disposable rocket using Space Shuttle engines that will cost over a billion dollars per launch. It's not lack of money that's keeping NASA from doing what SpaceX has done.

(SLS is bigger than the Falcon Heavy, but it's smaller than the BFR. There's a good chance the BFR will fly before the SLS.)