Top level pitching leaving for higher levels is probably the main cause (there just isn't that many true aces out there), but there is much more to the story. A D3 Juco where I grew up is right in the middle of a local hotbed of baseball. There are many kids from this area that are notoriously under valued by recruiters/scouts. This leaves them little to no offers. Plus factor in that many come from lower income families, they just simply can't afford a big 4 year college.