Sure, Classic D&D tops all ability scores at 18 (no silly percentile strength systems, or allowing demi-humans to get a 19). d20 is theoretically limitless in how high a score can go, but usually can't start higher than 20.
But look what those scores do for you, in relation to the most common NON-COMBAT resolution systems for the systems in question.
In Classic D&D, the default is to roll 1d20 and roll lower than your score.
In d20, you roll 1d20, plus your ability bonus, compared to a Challenge Rating set by the DM, but with guidelines given in the books.
So a character with Str 10 in Classic D&D will succeed at most Strength based checks 50% of the time. With an 18 Str, you succeed 90% of the time.
In d20, the default CR is 15. In order to get a 50% chance on that d20 roll at level 1, you need that maximum starting Str of 20 (+5 bonus).
Of course I'm using Strength as an example, because it's the first ability listed in both editions, making it a convenient point of reference, but this applies to all of the ability scores.
Of course, there are some sub-systems in Classic that don't follow the above pattern (2d6 Reaction rolls, with at best a +2 modifier for Charisma, for example, work on a bell-curve, but the math on that one still, I believe, favors Classic characters' chances.)
Funny. Some people, like my friend Alex, think that a Classic character with an average score of 9-12 is worthless. They've been deluded by AD&D, where you need a 15+ to get a bonus, and from d20 where you have no limit on your potential. The lower scores of Classic are actually more heroic.
And on the flip side, seen from this perspective, d20 adventures now seem a lot more challenging.