I suspect the question of ability scores mattering in AD&D is really that only HIGH scores mattered much in AD&D. Dexterity for example needed you to get a 15 before you saw an increase in the meaningful stat (AC adjustment). Constitution as well I think required a 15 before you got a bonus to hit points.
So rolling your stats more often that not would produce lots of numbers that wouldn't have any effect on your stats regardless of where you put them. You rolled one (maybe 2) stats over 15, they got put in your primary score(s) for what you are playing... the other 4/5 that were mediocre? Didn't matter where they went cause they'd have no appreciable effect.
Whereas in 3/4/5E you get a bump in modifier starting at 12, so most of your rolls will have a numeric impact on where you place it.
So rolling your stats more often that not would produce lots of numbers that wouldn't have any effect on your stats regardless of where you put them. You rolled one (maybe 2) stats over 15, they got put in your primary score(s) for what you are playing... the other 4/5 that were mediocre? Didn't matter where they went cause they'd have no appreciable effect.
Whereas in 3/4/5E you get a bump in modifier starting at 12, so most of your rolls will have a numeric impact on where you place it.