Comment by seanmcdirmid

Two reasons:

1. Back when I was in CS, the department was much smaller than it was today. This was after the 80s programmer crash, and so they just didn't have enough professors or resources to teach as many students as were interested in being CS majors.

2. You are able to teach to a higher standard if you've filtered the students that enter your department. You can have one or two people fail OS rather than half the class. At some point, it is reasonable to see if the students are committed, and they get to prove themselves a couple of years after high schools, which worked better for people like me who weren't very accomplished until they get to college.