To add to View From The Ground's list of possibly plausible reasons:
1. The economic feedback from the risk of losing hundreds of passengers (at greater than US $1 million per person likely liability regardless of actual fault) is higher than that for cargo flights of the same aircraft type. Despite all "safety first" claims, money talks.
2. Lower political pressure on the regulatory authorities regarding risks to a small crew vs. risks to hundreds of voters could mean a lighter supervisory hand in some aspects?