Market saturation!

People have what they need now, and no amount of enticement from the manufacturers will change that!

Back in the early part of the past decade, cameras whilst good, seemingly still had a lot of room to 'get better'. (better as in performance in an overall manner of speaking, not just one aspect of it)

So as cameras, and camera sensor tech surged, so did the rate of it's growth. That is, last years camera is really no better than this years!
But! ... 2006's camera wasn't half as good as 2007's camera.

So, in the last decade people saw a reason to buy 'this years' camera, and either stop using last years camera, or sell it off ....

If they want to sell more .. then with market saturation being what it is ... the only way to achieve higher sales is via lower price points.

Smartphone sales have peaked, and now in a slow decline too. It's paralleling what happened to the digital camera market with about a 6 year(or so) lag.
But the phone market has many cheaper option manufacturers which are in effect holding up it's sales volumes.
No such vendor exists in the camera market space. ie. there are no Huawei/OPPO/Xaomi type half decent quality manufacturer of cameras, so it makes sense that overall volume of the market is declining.