At schools with optional coop programs most students don’t participate because they feel that they will be missing out on something while working on coop: friends, classmates, needed courses etc. The school’s culture is geared to the traditional two semester and summer off pattern.
Even at a coop school like Northeastern many prospective applicants ask how they will be able to “make up” what they missed while on coop. At Northeastern the open houses and admitted students days emphasize how the campus culture revolves around coop and that nothing is “missed”.