I understood this to be due to the possibility of pressurised GA traffic up to 18,000’.
What is the the connection with pressurization? I have often flown unpressurized aircraft, some with no transponder or ADS-B Out, up to 18,000 ft and much higher a few times.
(Before anyone jumps on "more than 18,000 ft without a transponder" research "glider wave window").