It's boring to say it's just age, but it's just age. Blake was 21 as a rookie, Yao was 22 (and boosted by the Chinese vote, he made the team every year of his career, even his last season when he scored 51 total points all year). Only three guys in last year's lottery were 21 or older during this past season, and I don't think it's a giant coincidence that that group included two of the more impactful rookies (Murray, Jalen Williams and Agbaji). A year or two at that age is just huge for physical development, 25+ years ago top prospects routinely played 3-4 years in college before going pro, it makes perfect sense that with those same sorts of guys playing just 1-2 years in college now, they aren't quite as ready for the league as rookies.