Certification during emergency descent is a limiting factor, even with ADM. In Europe RAD is often preventing very high cruising level ( agreement between ATC centers).
At these very high levels, residual thrust is very low, hence you need to be light, since you are looking for the minimum time between two points, you surf the winds. And except for a smoother ride,(or Very long range ops) very high cruising level may not be the best.
Long Range, mean Low and Slow initially, so you might see a plane in the low 40 initially that would climb to 470/490 at the end of the cruise.
For FAA, you have O2 considerations as well..
But as far as the airframes are concerned, the main trigger is really , like said above, cost of certification and manufacturing, the delta P for a 4000 ft cabin at FL510....hmmm pressure cooker..
Cosmic radiation detectors are only required above FL510