Ever since airlines started charging for checked bags, there's a significant incentive to board early no matter your seating, since the overhead bins universally fill up about 1/2 to 2/3 of the way into boarding.
When you see the people checking seemingly all their worldly possessions, it's mostly families.
I don't doubt there's some effect at the margins but people mostly just prefer to carry-on if they can.
I partly blame roll-a-boards. They take up more space and are less flexible than soft luggage.
I also think that in the "old" days, carry-on garment bags notwithstanding, business travelers were far more inclined to check because they needed a lot more clothing to be properly dressed. I used to check luggage when I traveled on business. I haven't done so, barring unusual circumstances (gear for personal travel or commercial materials), in well over a decade.