the pricing is a big diff. the big prime contractors tend to cost more per payload mass than SpaceX. plus the primes do pricing based on "cost plus" which means a general incentive to increase costs in order to increase profits. SpaceX is doing firm fixed price quotes, and builds their profit margin into that (at least in long run, after R&D costs recovered), and eats risk, as you mentioned.
Also, and this is big, frankly SpaceX is operated in a much more nimble and entrepreneurial fashion. The primes, while having lots of very smart and well-meaning people working for them, have BDC-itis. Musk wants to dramatically improve the world and the species, and put a colony on Mars. The leadership of the primes seem to just want to make money. Musk/SpaceX is more exciting and compelling for a lot of folks, plus they're more likely to lead to spin-off missions down the road.
Also, and this is big, frankly SpaceX is operated in a much more nimble and entrepreneurial fashion. The primes, while having lots of very smart and well-meaning people working for them, have BDC-itis. Musk wants to dramatically improve the world and the species, and put a colony on Mars. The leadership of the primes seem to just want to make money. Musk/SpaceX is more exciting and compelling for a lot of folks, plus they're more likely to lead to spin-off missions down the road.