I think everyone trying to name a single first order reason for more power is wrong. I think many little innovations occurred and became possible for various reasons.

1. better computer control 2. advances in variable cam timing mechanisms 3. direct injection 4. coil on plug ignition systems 5. improvements in sensors (knock sensors, mass air, oxygen, etc) 6. improvements in machining/forging etc leading to tighter tolerances 7. improvements in engine oils

probably dozens of other things