Some of the offending passengers may be clear headed and simply making a selfish but conscious decision to lug their carryon with them. Others are likely falling victim to a number of psychological factors come into play in panic situations - even those of us who sit here at the keyboard thinking "surely I would/wouldn't do this or that" may or may not react the same in an actual emergency. More should be done to formally study these effects - as they come up in pretty much every such incident - and come up with better ways to address the problem.

Plenty of good articles on the subject such as this:

Let it go, people | Flight Safety Australia