Various reasons: Good: NASA started SLS years ago, and had no idea that SpaceX would be so successful. They also didn't know SLS would take as much time and mo

Various reasons: Good: NASA started SLS years ago, and had no idea that SpaceX would be so successful. They also didn't know SLS would take as much time and money as it has. Bad: NASA is at the mercy of Congress and SLS has a lot of political support. It spreads jobs over a lot of districts and pays big money to major defense contractors. At this point it's a huge sunk cost and embarrassing to cancel. Ugly: Those contractors work on cost-plus contracts so they have little incentive to make a cheap launcher. To illustrate, the SLS uses Space Shuttle main engines, which cost several times as much per unit thrust as the Saturn V's engines. That was worthwhile for the Shuttle since it reused them. Naturally, NASA's contractors thought they were perfect engines to throw away on every flight.