Well I think it's driven by the fact that given the limited scholarships, there are a ton of kids who are good enough to actually get recruited by schools (and could play at them) but not good enough to get any money from them. We've had a number of men's tennis players over the years who came in as walkons and turned out to be very good players for us, including at least one on the team right now. Whereas in basketball, only very rarely is a school going to recruit and regularly play a kid that it's not going to give a scholarship to. So you have to get your walkons from students already on campus.
And while taking walkons through tryouts may in some respect be a nod to the community and strategy for raising GPA, in basketball and football the primary reason is so that you can have a scout team to run the other team's plays during practice.