That pork pays for a lot of engineering salaries in places like Alabama. Senators will fight tooth and nail to keep it.

With only SpaceX, there's a risk of the technology becoming expensive or not doing what the government wants. This hedge de-risks that scenario and is worth the cost alone.

Once we have several commercial rocket families to choose from, we can reconsider. In the meantime, we've already paid for SLS and it isn't going away - and that's not a sunk cost fallacy. There are mission specific payloads SLS will be better suited for.