TLDR; The GAO identified the culprits as “risky management decisions, unforeseen technical challenges—some avoidable and some not—and workmanship errors.” Look

TLDR; The GAO identified the culprits as “risky management decisions, unforeseen technical challenges—some avoidable and some not—and workmanship errors.” Looks like nothing we didn't knew. NASA wants very high success rates on missions which leads to spending enormous sums to drive down probability of failures. On the other hand, Musk can blow up rockets dozens of times and keep moving at a fraction of the cost. The second biggest factor is simply the unknowns. It's not that we have launched giant telescopes in space dozen times and have became very good at predicting costs.