Probable reasons which may make sense and lead some to fly across the country:
1. Event director's ability to recruit athletes
2. Win the mile at Dr. Sander = guaranteed entry into Millrose Mile (Gold / "A" level)
3. Marketing of event. Dr. Sander did a decent job announcing athletes and promoting the event
4. The Dr. Sander meet has been around much longer (years before the Podium in Spokane was built) - name recognition/familiarity/etc.
There are several reasons some smaller events with smaller budgets attract better competition than those with more prize money, etc. It's a positive thing that Lilac is creating more opportunities and raising the bar but calling out coaches and agents for sending athletes to a different meet is a jab below the belt imo. Work harder and make your meet more appealing and they'll probably come there next time.