It's a combination of things. Pocket calculators reached pretty much their apex of development some time back. The HP50G you can get today is a very incremental enhancement to the original HP48 that was available twenty years ago. In HP's defense, the modern $150 top-end price point is much lower than the HP48SX and, later, HP48G that popped up when I was in college.
On the other hand, manufacturers worked hard to cut deals with school boards and textbook publishers, and to overcome the idea that math education aided by a calculator wasn't really math education at all. So in particular we started seeing things like textbooks referring specifically to the TI-83, practically obliging school districts to buy truckloads of them.
So now you've got a stagnant technology with a captive audience. And that's why prices don't change.