The goal of the Apollo missions was to land humans on the Moon and return them to Earth. That requires a surprisingly large delta-V capability. (In terms of delta-V, is easier to land a vehicle on Mars and leave it there than it is to land a vehicle on the Moon and leave it there.) The Space Launch System plus the Orion capsule do not have the delta-V needed to land a vehicle on the Moon and return it to Earth. SLS plus Orion does not even have the delta-V needed to insert into and later return from low lunar orbit.
On the other hand, the Orion capsule has an extremely capable environmental control and life support system (ECLSS), capable of supporting a crew of six for 21 days. It can afford to make a leisurely trip. The Apollo vehicles did not have this luxury as they had a rather limited ECLSS capabilities. The outbound and inbound trajectories to and from the Moon were intentionally designed to be suboptimal with regard to delta-V so as to make the trip take less time. Had the Apollo vehicles used optimal trajectories (from the perspective of delta-V) the result would have been dead bodies returning to Earth.
The Apollo missions had the spacecraft immediately enter low lunar orbit upon getting close to the Moon. The Artemis-1 mission will instead use close lunar approach as a powered gravity assist and days later inject the vehicle into a distant retrograde orbit (DRO) about the Moon. The drift from closest approach to the Moon to DRO insertion will take a few days. Targeting lunar DRO as opposed to targeting low lunar orbit reduces the required delta-V to something the SLS+Orion can handle. (Subsequent missions will place the vehicle into a near-rectilinear halo orbit, with similar delta-V requirements.) The greater time needed for a fuel-optimal trajectory to lunar periapsis followed by a drift to DRO altitude combine to make the outgoing trip take eight days rather than a bit more than the three days used by the Apollo missions.