I guessing part of the reason is the pressurization of the aircraft. At FL350 I get a cabin altitude of around 8000 ft. The press controller is nearly maxed out at 7.76 psid and the emergency pressure relief valves will open at 8.06 psid, so I guess I couldn't go much higher without sucking an O2 hose. I assume designing an airplane that can handle the pressure without popping like a balloon presents it's own challenges and the fix may not be cost effective.