Why is 51,000 ft a common service ceiling for business jets? According to this article on aerospace-technology.com, many of the larger business jets seem to have a service ceiling of 51,000 feet. Other sources verify this trend.

What is the reason for this, if anything specific can be named? Some possibilities that I came up with are:


engineering considerations (limitations of materials, pressurization, systems capabilities etc.)
safety, i.e. time of usefull consciousness getting too short higher up (being less than 10 s @ 50,000ft)
too little to be gained going higher (no congestion anymore at that alt anyway)
an industry agreement (based on any of previous and/or other factors), in the same manner as european automakers' pact to limit the max speed of cars to 250km/h (excluding some model)
regulatory factors


Other common "steps" appear to be 45,000 ft and 41,000, with smaller models. Nice round numbers all, but surely there has to be some reason for the magical 51,000 ft figure. Interestingly, the Learjet 85 was designed to have a service ceiling of "only" 49,000 ft, and the program got cancelled at least partially due to poor sales. It seems it was simply lacking the last 2000 ft...



Examples of business jets with 51000ft service ceiling:

Cessna Citation X

Falcon 7X, 8X

Global  5000, 6000 and 7500

Gulfstream G550, G600, G650 ER

Learjet 40, 45, 60, 70/75

etc...
If for no other reason, its likely because there is a practical maximum descent speed for a given airframe and aircraft must be able to descend from their maximum cruising altitude to 10,000 ft. in 10 minutes or less. Although under part 135 (how many of these planes are likely flown) the regulations have a few variations you can have less oxygen on board if you can descend in 4 minutes. 

At 51,000 ft. you would need to lose 41,000 ft in 10 minutes or even 4 minutes. That is a descent rate of either 4100 ft/min or a blistering 10,250 ft/min which likely yields a speed at or near Vne for most of these airframes. 

This Gulfstream pilot discusses a bit of it from a practical standpoint here.