Literally only 3.0 and 3.5. B/X went up to 14, BECMI went up 36, and AD&D went up as far as anyone wanted to take it, as long as you were human and not a monk. Late AD&D did "epic" stuff starting as early as 9th level, but only really ramped it up in Dark Sun with the Dragon Kings supplement.Since D&D had used 20 levels for nearly every edition, and 4e was so different than every other edition, I imagine the thought process was that people would see it as going back to normal.