There are a few necessary activities and schedule considerations that contributed to needing 18 hours between undock and entry.
Loading the Crew Dragon with returning (“down-mass”) gear, readying the capsule for undocking and maneuvers, and prepping the astronauts themselves is a full day of work, even for the three U.S. astronauts that were on the ISS. There were likely tests and measurements to be done for this first crewed flight that also slowed down the process. Bob and Doug needed their 8 hours of crew sleep period so they would not be sleep deprived for the entry, descent, and landing.
The trajectory that the undocking capsule follows, detailed in this Scott Manley video, requires about a full orbit (1.5 hours) of careful maneuvering. This is due to two factors:
- The thruster firings must be small and off-axis in order to avoid thruster plumes impinging on the ISS, especially on its solar panels.
- From mission rules, at no point can the capsule bring itself into a trajectory that has a chance of hitting the station within the next four orbits.
An elliptical orbital plane is fixed in an inertial reference frame, which means the non-inertial rotation of the Earth is independent of the orbit. This means the point where an orbit crosses above the equator moves 360 degrees every sidereal day (a little less than 24 hours). In order to land in a particular location without an obscene amount of fuel use, you must wait for your orbit to phase close to above that point AND plan ahead so you reach your mark to start your descent on time. The phase aligns twice per sidereal day, so the schedule must be planned around that.
This is speculation, but I think Bob and Doug were tasked with more orbital maneuver testing and checkout of the Crew Dragon, which takes some more time. Also, there is some buffer time built into the schedule to handle minor unexpected events.