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.
Not to mention there's risk in NASA putting its Orion dreams on something thats currently unproven. If the publish saw Orion blow up today then that could prove disastrous for the SLS program.