Because they shutdown the production lines capable of making them for $47 million over a decade ago, and it costs a lot of money to recreate them and get the to

Because they shutdown the production lines capable of making them for $47 million over a decade ago, and it costs a lot of money to recreate them and get the tooling and personnel needed. It's also the single most expensive rocket ever. It was designed to be the best, economics were ignored because they had the budget to do everything optimally. In terms of basic specs, RS-25 is comparable to Raptor with superior specific impulse, and is a highly proven 45 year old design. Reliability is really the most important stat, given the cost of the payload and other parts of the rocket. Obviously Space X spent a lot of time thinking about economics, with great results, but going for the expensive but sure bet option made a ton of sense for SLS. Merlin is much simpler and smaller and not really comparable to the other two. The RS-25 got new life with SLS, I would expect future NASA craft to use Space X engines.