Quite simply....
Regarding takeoff performance, in the earlier days of jet transport flying, the screen height during takeoff was lowered from the standard fifty feet (for piston transports) to 35 feet because...many of the early model jet transports at the time (B707's/DC8's) could not meet the fifty foot requirement with the runway/clearwar lengths available at the time.
So, alter the regs (CAR4b) so they do fit.
Simple as that.