The Space Shuttle was incredibly unsafe design, and it evolved fairly little during it's life. Mixing unthrottable and unstoppable solid rockets with liquid rockets was never a good idea. The massive external fuel tank was vulnerable. The tile system was an achilles heel, and extending the wingspan to force air force payloads on the shuttle made reentry even more dangerous. Finally carrying a huge space plane every flight eliminated more than 80% of its payload capacity.

Falcon 9s and Dragons are an evolution of tried and true rocket engineering principles, even capsules are much safer at reentry than the shuttle. Delta showed that type can be launched with a high degree of safety. SpaceX has the advantage of getting returned equipment back from space to learn from and improve.