Spacex also had the very small advantage of all the basic science being done for them for free. The constraint in the NASA golden years wasn't getting it done cheaply, it was getting it done at all.
Rockets haven't changed much since the 60s. This doesn't explain all of the launches since the Space Shuttle, for example. Furthermore, SpaceX clearly innovated with their reusable rockets, but this is exactly the kind of cost constraint that commercial ventures prioritize which government ventures often don't.
