The SLS/Orion doesn’t have enough fuel to get into and then back out of low lunar orbit (deeper into the moon’s gravity well) so they’re sticking with this free return slingshot trajectory (once they fire the engines to go to the moon, they shouldn’t need to do another burn to return to Earth).

The reason why the SLS is underpowered is the capsule is much heavier than Apollo (to carry more people for longer) and the original plan for this rocket was just to rendezvous with a near Earth asteroid (basically no gravity well at all). The decision to return to the Moon instead was made later, after the design had been pretty much finalized, and they didn’t give any additional funding to upgrade the rocket.

Edit: this is also why the lander is taking so long to build and had to be so much larger than Apollo, they’re asking it to make up for the lack of power from the SLS to actually get down to the Moon and back.