According to the Wrestling Observer WWE had originally planned to use Sting at Summerslam and had instructed him to get into wrestling shape (which he did, impressively). However they decided not to use him because the card is already so stacked.
It’s now believed that he will wrestle at a later date, possibly September’s Night of Champions, so they can use his star power to sell the show. The event takes place in Texas which is where Sting currently resides.
The original plan was to include him in a tag match against the Wyatts. If Erick Rowan is back by then it could be a six man featuring Sting, Roman Reigns, and Dean Ambrose; against Bray Wyatt, Luke Harper, and Erick Rowan.
Sting will still be doing signings at Summerslam but he isn’t expected to appear on camera.