For most students, the Greek scene at Yale comprises only a fraction of student social life. Fraternities are know primarily for their parties, complete with sticky floors, stepped-on toes, and red plastic cups. Many of the fraternities serve as clubs for varsity athletes (such as Alpha Delta Phi and the Yale varsity lacrosse team) and most of the houses (if the group has one) are run-down. Sororities are even less visible than the fraternities. They act primarily as emotional, intellectual, and social support systems for their members.
The reason Greek life remains less popular at Yale is most likely the strength of the residential college system; students are already inherantly part of a familial group that offers support, social opportunities, and camaraderie. Thus, frats and sororities are stuck in the background because they simply aren't necessary. That said, there are many students who enjoy the Greek system and make it an integral part of their life at Yale. Essentially, Greek life is only one of the hundreds of ways to get connected and have fun on campus.