Because filling up with more pros would draw more spectators. A certain percentage of spectators buy cue raffle tickets, book rooms, gamble, purchase from the concession stand, and eat at Turning Stone restaurants. That may entice Turning Stone to keep the event there and keep adding that 25K (I would guess that's the TS adding that 25K, not sure though).
We live a little less than 90 minutes drive east of Turning Stone, and we try to make the trip for a full day whenever they have this tournament. I have pool friends that play in the event, and we went up on Friday this time, so we got to watch them play in-person. Just as you reference, we bought raffle tickets, bought something from a vendor, we gambled a bit, and we bought food from the concession stand (two different times). We were there from about noon, until after the 10:00pm round that night. We usually do eat dinner at either the steakhouse or the tavern, but with people I knew still playing, we went with the snack bar concession, and ate while we watched. Food from the snack bar was actually pretty good and about half price compared to the food court downstairs.
I also got an email from Turning Stone asking about my visit, probably due to my using my player's card while gambling, and not having been there in a long time. It asked the usual service related questions, but it also asked what the purpose of my visit was, and I made it clear that the pool tournament was the main purpose. Of late, I have been seeing a lot of adverts about working at TS in a number of capacities, full or part-time, so they might be hurting for employees, and looking to deal with some other problems. We walked through the casino when we first got there, and it was not at all busy, with very few gaming tables open, aside from the slots. A bit busier later at about 5:30, with a few more tables open, and it looked like business as usual with everything open at about midnight when we left. All that aside, there is a great deal of construction going on around the casino and hotel. It's just about surrounded by fenced off construction sites, and I doubt that they are more parking garages.
Mike did announce something about him still being in negotiations with the casino for the next event around Labor Day, and they didn't have a date set for that. Whether that is because there are other activities at the casino around then, or if they are rethinking their support of the event, who knows. I think that the event center is a great venue for this event, and the stands on either side of the room have ample comfortable seating to watch whatever match you'd like to see, and still be able to watch a number of other matches at the same time. So as a spectator, I think it works incredibly well, and offers a great deal beyond the pool itself, but I know nothing about the business side of things.