It's an aerodynamics problem. SpaceX has designed both of their Dragon capsules to be inherently stable for reentry; should the capsule not be able to orient itself actively due to a Draco engine failure - it will passively orient itself during reentry with the blunt heatshield pointing forward, into the velocity vector. This minimizes risk and ensures the heatshield is always facing into the atmosphere for descent. This is an inherent property of the truncated cone reentry body.
This design has one drawback however: if you have a scenario where Falcon 9 encounters a failure mode that risks the lives of the astronauts on board the Dragon, and you need to escape from the rocket, you want to do so as quickly as possible and encounter the least aerodynamic resistance as you can. This means you need to point nosecone forward, and heatshield back, which is precisely opposite to the aerodynamic design of Dragon.
The solution SpaceX chose was to keep the trunk on during the abort. Note that the trunk itself does not keep the capsule pointing forwards, but rather the fins on the trunk do. They sculpt the airflow around the capsule in such a way that it is stable nosecone forward into the velocity vector.
Dragon immediately begins to rotate post-trunk deploy, prior to drogue chute jettison.
You can see all of this during the Pad Abort video. As soon as the trunk deploys and separates from the vehicle, the capsule immediately begins to rotate, trying to stabilize with the heatshield forward.
Additionally, the trunk is incredibly lightweight. Dragon weighs in at around 8000-9000kg. The trunk is less than 1000kg. A minor drawback, sure, but who cares when you're saving the lives of the crew inside.