Get enough cavities in your teeth and they fall out, making crowding a lesser problem than the lack of teeth.
Perhaps tooth crowding is a problem now because people have better dental care?
No, because tooth crowding is a severe health-threatening problem long before one gets to the age when teeth are falling out, plus even people with lost teeth often get life-threatening complications from impacted wisdom teeth -- they quite simply emerge too far back in the jaw. Too far back for our *modern* jaws, anyway -- apes' wisdom teeth fit just *fine* in their long jaws. Hint, hint.