Why do upperclassmen move on?

You're okay rushing as a soph, plenty of enthusiasm still left for joining and you would find a close friend group in your nm class. Harder to do that as a new member if you're a junior.

Junior actives shift into becoming chapter officers and exec, but are otherwise begining to realize they have seen and done it all in Greek life so don't participate in every single thing as much as the freshmen and sophs do. They have solidified their small close friend groups that include some of their chapter sisters as well as members of other sororities too, and also close friends in fraternities. They are mostly not in the house or dorms but have apartments with or near their friends. Also school gets harder junior year and you get into the heart of your major's classes. Seniors are all this and more and are focused on graduation and what comes next.

You get out of anything what you put into it. If you want to remain involved, you will put forth the effort.