Not sure why the pre-79 season premieres were in February rather than September. But back then, I'm not sure that PBS had seasons starting in September, since none of the PBS kids shows that I know of started in September.
Also, note that when they started having printed schedules in 1984, the printed schedules were from September to August (except for the first one from January to August 1984).
What is interesting is that 1971 was when most of our holidays were moved to Mondays, including Presidents Day, and the 1971 - 76 season premieres were all on Presidents Day. I assume that was intentional. Interestingly, the 1968 season began on what would have been Presidents Day, but it hadn't been moved to a Monday yet. The 1969 and 1970 seasons both started 51 weeks later, probably since one week (probably 1-5) didn't reair.
What I often wondered was how the post-75 debut dates were selected. Some of them followed a pattern, but others seem totally random.
Maybe somebody here (such as Tim) can ask FCI whether or not anybody remembers why the pre-79 season premiers were in February rather than September. I doubt anybody would remember at this point, but it's worth a try.