NASA has been directed by Congress to use the remaining Space Shuttle RS25 engines on SLS. There aren't that many RS25's left, so Artemis requires that they make the most of each launch. Getting more RS25's produced is one of those "nobody's made them in a long time and it would be terribly expensive and time-consuming to do so" type of situations.

correction: there are 16 RS25's left, but production has begun on more for the Artemis V mission. However, production is slow so they can't just yeet SLS's into space and test rapidly.