It's a matter of arithmetic. Fall Semester consists of fifteen weeks of instruction, a week of finals, and a break week at Thanksgiving. Total, seventeen weeks; if you want to finish that before Christmas, you've got to start in August.

When I first came here, we were on the quarter system; since a quarter contains only ten weeks of instruction, we were able to start in September. (Spring Quarter, however, ended about a month later than Spring Semester does.)