The competition and level of coaches in DI is just much, much higher than DIII. As is the competition for recruits. It's just not comparable.
So yes, BCS teams have certain advantages. But teams are also cannibalizing each other during recruiting, and you're not in a division filled with loads of sims and less experienced coaches. (That's not to say that all DIII coaches are weaker/less experienced of course, but on the whole I think it's a very fair statement.)
oldave, to response to your last comment, DIII teams don't have built-in advantages, they have advantages that they've earned, and that's the difference. That said, I don't think it's much of an argument that doing it at DIII is easier.