Near sports betting and racing areas - People that bet on sports and racing usually like to have a bit more of a quiet atmosphere to hear the race calls or the commentators instead of the constant clinking of coins or tokens going through the machines. Sports betters usually bet big, like table players, so the casinos would prefer they have their own area away from the noise of the slot machines to keep them there (and betting) longer. Putting the well-playing machines around there would make more noise, and could easily drive away the sports betters. Casinos want everyone to feel comfortable and happy in their own place so they'll stay longer and bet more.
