Earlier quoted context omitted.
Arguably looking out the window and realizing they were in the Hudson River might have lead them to realize they weren't going to be swimming with their wheelie.
The U.S. is a very punitive society and I think if you were to find yourself in the tarmac after an evacuation holding onto your carry on, you’d at minimum be getting a large fine for disobeying a flight attendant and possibly more. Not being in the Hudson isn’t going to change the fact that you are ending up in the middle of nowhere once you are off the plane.
What I’ve read, the reason people taking their carry-on at least partly is because, when having to get out of a plane in a stressful situation, they won’t think “what shall I do now.”, but just activate their “deboard plane” program, and that involves getting their bags before getting out of the plane.