I am led to believe (by an engineer who worked on our PW125B's) that it is due to the engine spools potentially seizing if an engine flamed out on takeoff. This would then prevent an in-flight restart for quite some time. This comes about due to the uneven cooling of various sections of the spools and the itt limit is there to stop that from happening. (Ie cooler OAT, then shock cooling would happen from a lower ITT, if higher OAT then less shock cooling so you can run it hotter)
Whether that is actually the case or not I would be very keen to find out too!!
