TES, the engine runs out of puff for the same reason humans do at altitude. An engine designed to process fuel/air efficiently at 35000 will be unlikely to have the compressor/turbine aerodynamic properties, nor the fuel flow to operate in the more rarified 50000 ft alt. You can imagine the inefficiency of the Olympus on the Concorde if it were cruising at 39000 ft (the norm for most modern airliners today). It would use fuel at an alarming rate compared to a CF6/4060/211 I should imagine.
However, as alluded by 9freighter, the real reason is the pressurization loads required to keep the cabin at 8000ft would need a vastly heavy structure to cope. This is the very reason the fuselage dia on the Concorde is about the same as an F27. If you increase the internal volume of the pressure vessel at the same pressure, you have the X psi's acting on more square inches of area, requiring a much stronger structure/thicker skin to withstand the load.
For the sake of simplicity here, I am ignoring the supersonic wave form effects and surface heat a large physical size fuselage would need to have catered for it.
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When all else fails, read the manual!