As mentioned briefly in this question, The Russian Soyuz has been using a sub-4 hour path to ISS. Why the long ride from Canaveral? Or is the 19-hour timeframe typical of all launches prior to the "Express" Soyuz methood?
To achieve a 4 hour docking time, you have to really have everything working perfectly. Crew Dragon is still a very experimental spacecraft, and won't do such a quick docking for a few reasons. Of some note, the first mission took around 26 hours to dock with the ISS after launch.
- They will want to test the various systems prior to docking, making sure that everything is working perfectly. Some of these things can't be done at the ISS. Checking that comms are working well, thrusters, etc, all will be done. Keep in mind, the main goal of the mission is to ensure the spacecraft works right, they will have to check those things out, especially the ones that require humans in the loop.
- The exact accuracy of orbital injection, etc isn't super precisely known yet. In order to do a 4 hour dock, you have to have a nearly perfect insertion. Some Soyuz missions have been delayed to as long as 2 days when the insertion isn't perfect.
- They will just want to take things slow in general, to make sure that rushing things doesn't lead to a new problem, etc.
- The crew will have been up for a long period of time. They will want to let the crew rest before doing the highly sensitive docking maneuver.
They may be able to dock sooner, but this is the first mission, they are going to take things slow. Also of some note, the planned timeline to orbit requires 3 major burns performed on orbit. Roughly half of the time is spent coasting after the final major burn, the other half is spent waiting for the best time for the 3 major burns.