Lower volume, higher unit price. Also cameras and lenses are (generally) much much better than they used to be. Perhaps people are willing to pay more for a better thing.
The cause and effect you imply here are backwards I think. The products are better to justify their higher price, not higher priced because they're better. The first sentence has it right - if you're faced with lower volumes, the best way to cope is by raising prices.
The products are better because of steady improvement in precision manufacturing. Sensors and lenses have both come a long way, the latter is mostly a benefit of cheaper precision steppers.
The prices are higher because phones have taken the whole entry tier of the market out, pretty much. There's not a lot of point in a good digital camera with one noninterchangeable zoom lens, when pictures from a phone camera are almost as high quality, and action cameras like the GoPro also chipped away at that product.
Cameras are a specialty product now, with APS-C and 3/4 as the entry level. People who want that kind of control and quality are willing to pay for it, and there aren't as many of us as there used to be.