Different requirements at the time. But one thing that most people don't know, is that federal law basically says that a maximum of 1000 cadets can graduate and commission per year. If there's more than that, they aren't going to usually deny a graduate from commissioning, but they have to explain to air force, DOD, and congress, what they are doing to offset that number. Usually, they allow them to bring in less the next year or so, believing that the "AVERAGE" graduation rate won't exceed 1000.
FWIW: The law also states, that the academy isn't suppose to exceed 4400 cadets "AT" the academy. Again, they arent going to throw anyone out normally, but they have to show how they will compensate. That's why one year they might bring in 1200-1300 cadets, and the next year they may only bring in 1100.
Also, prior to 2008 and the economy "Melt-Down", choosing the military and the academy as a "Job" wasn't as important. Unemployment was lower and outside jobs were more abundant. As such, the academy would offer approximately 1700 appointments, KNOWING that approximately 500 would turn them down. (Yes, believe it or not, not everyone who applies to the academy have it as their #1 choice for college). For many, it's their #2, 3, 4 or 5. If they get their #1 or such, they turn down the academy. Well, after 2008, unemployment skyrocketed. The military was a more sure thing for a job. They realized they couldn't offer 1700 appointments, because not as many would turn it down. Hence, the reason for more of a rolling appointment process whereby some appointments are given out, regroup, some more given out, regroup again, etc. until they've given out the number they want.
Sorry for the novel, but a lot of people have wondered and asked my "HOW" the academy decides how many appointments they will give out each year.