i'm gonna go with transcendent talent and rule-changes (which people have said) explaining why we don't have a payton or a harper.
i also wonder if there is a generational thing. an off the top of the head hypothesis: in the mid 2000s, the need defender was a wing defender, not a PG stopper (think Bruce Bowen). now, PGs are much more influential to team offenses than before, but the reaction--building a defensive stopper at PG--hasn't caught up. Plus, you don't want to cripple your offense for the sake of your defense. so you see the PG stopper playing as a 2 guard (Bradley) so the PG can still run an elite offense on the other end of the court.
----------- It's only funny till someone gets mad. Then it's hilarious.