Same reason a piston engine has a lower fuel consumption at altitude: there is less oxygen available for combustion, thus less fuel required to achieve proper stoichiemetric combustion. The turbine engine behaves rather like a normally aspirated, fixed-pitch piston engine. As such it is producing much less power at altitude. This is compensated for by the low atmospheric density, the airplane "indicates" 300, but it is actually travelling at 500 kts "true" airspeed. This what you were looking for?