The power potential of the gas producing section of the engine is totally dependent on the density of the air it is operating in. When air is dense, on a cool day at sea level, for example, the turbine section loafs along. The compressor has plenty of air to work with so it feeds the burner section its maximum charge of air using only low rpm and relatively low compression ratios. But when the air is less dense, at high altitude, or when air temperature is above ISA, the compressor struggles to ram the same air charge into the burner. The air is hotter exiting the compressor and burns hotter. The compressor must spin faster to do its work. And at some point the density of the air available to the compressor just isn't enough for it to deliver the full charge of air into the burner before reaching the rpm limits, or the temperature limits, or both.