Aerobat77:
now we can talk why it is so a a turbine engine.
Turbine engines utilise air for cooling of the combustion chamber and the turbines themselves. ITT will rise due to the reduced mass flow rate of air going to the combustion chamber as you climb.
Less mass of air going into combustion chamber, but same mass required for combustion itself = less left over for cooling. I'm not saying this is the only cause (I have a mere pilot's understanding of turbines) but it is the only cause I was taught at the Royal Naval Flying Training School. It is important to helicopter pilots because you can become power limited in the hover due to turbine temp before you reach the service ceiliing - a factor in so called 'hot and high' operations.