Arrogance — and the story shows the exact moment the rot sets in.
Early Light wins because he treats every opponent as smarter than they appear and plans for failure. Post-L Light stops doing that: he assumes the SPK are inferior copies, delegates critical work to Mikami and Takada (two worshippers, not partners), and never builds contingencies for the notebook swap because he can't imagine being outplayed. Near even spells it out: he and Mello only won because they were two — Mello's recklessness forced errors that Near's caution alone never would have.
The 'bad luck' reading falls apart on inspection: every coincidence that sinks him (Mikami checking the fake notebook, Takada's kidnapping spiraling) is downstream of Light choosing fanatics he could dominate instead of allies who could tell him no. That's not the dice turning — that's the god complex selecting for its own blind spots.
Ohba has said L's death was the story's midpoint, not its climax, and the final arc is deliberately a mirror: Light becomes the establishment being hunted by young geniuses. He loses the way L lost — by being one step behind someone with nothing to lose.