Carry on bags are a challenge for airlines that are trying to depart on time. That’s become far more the case over the past 11 years since US airlines added checked bag fees (so passengers try to carry on more) and they’ve resisted adding flights (capacity discipline) so that planes are increasingly full.
It takes time to store carry on bags on the plane, but perhaps more importantly when passengers discover at the last minute that there’s no overhead bin space left it takes time to gate check their bags.