Like all tennis tournaments, there has to be a payoff. The overall winners of the summer circuit (singles and doubles) get entry into the ITA All-American tournament in October. The runners-up get into the qualifying at ITA AA.
Top 30 (or so) players will figure that they are getting into the ITA AA because of ranking. So, you can only attract the guys who are a little lower. Maybe they need to add more main draw, qualifying, and pre-qualifying spots based on Summer Circuit results to attract more top 100 but not top 30 players.