Delta IV Heavy (which EFT-1 flew on) is currently the world's most powerful rocket, capable of putting 30 tons into LEO, about twice the payload of the Falcon 9. The Falcon Heavy which is supposed to make its first flight sometime next year is supposed to carry about twice the payload of a Delta IV Heavy[1]. However, NASA could not choose it for a few very good reasons: A) it isn't ready to fly yet and B) NASA really really needed this mission to go perfectly, and so being on the first flight of a brand new, less tested rocket created too high a risk of program cancellation.
NASA was very concerned that if there were problems with this launch it would lead to program termination in the current budget environment. Especially given the fiasco of the Constellation program, and the fiasco of every program save two (ISS and STS) dating back to Apollo Applications and the Space Task Group, I can understand NASA's concern.
[1]: Note that when Musk originally announced this rocket as the Falcon 9 Heavy back in 2011, it was supposed to first fly in 2013, and all their website sas now is "Expected in 2015". Rocketry and space is HARD. Even for SpaceX.