Congressional mandates forcing NASA to use Space Shuttle components for the SLS that are de facto non-competitive, single-source requirements assuring contracts to existing Shuttle suppliers is contributing significantly to the costs.

For example, the NASA contract to Aerojet Rocketdyne to manufacture RS-25 engines is costing $146 million per engine. Each SLS launch will toss four of these engines in the ocean for a total cost $580 million. And that's just the engines of the core stage. For the cost of a single engine, they could have instead purchased an entire Falcon Heavy launch, which has two-thirds of the SLS lift capacity (and has to obey the same laws of physics as SLS).

SLS is the price NASA had to pay to get Congress off its back.

Why would the US congress mandate stuff like this?

The more dollars and jobs you can claim you brought to your district during the next election, the better.

This isn't just a NASA thing, a NASA contract is probably pretty small compared to getting a new military base, or keeping one open that the military wants to close.

Because US congress is made up of elected representatives, and each representative is incentivized to do so by the desire for re-election.

Because it benefits them (in kickbacks from contractors, for example) and because they can get away with it.

Having a well functioning space program? Doesn't benefit them much or at all.

Ultimately, it's because voters don't call them on their BS. If you've been a NASA fan, encouraging NASA funding regardless of what it's for, you're part of this problem.

I'm not an US citizen. I can't call out US politicians on anything because I don't have a vote.

It's more that the cost in money doesn't come from the cost in energy.

Why do you think the cost of launch is due to the cost of energy?