Loss history definitely plays a role in the cost of insurance, but the overall trend is unambiguously downward thanks to improving aviation safety. There are plenty of people willing to bet that this trend will continue over the long term, despite the odd outlier.
At the same time, ultra-low interest rates mean there's a lot of money in financial markets looking for a better return. Plenty of this has found its way into insurance and reinsurance markets, and that's also keeping premium costs down.