#1 WWE doesn't want the show to go on for too long
While WWE has been delivering on the quality of the matches in the recent PPVs, one complaint the fans have had with the company is that the PPVs are way too long.
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WrestleMania this year was a shocking 7 hours of continuous WWE action, and while the fans enjoyed the show, it was a bit too much. Even 'secondary' PPVs like Extreme Rules or HIAC cross the 3-hour mark which drains the energy of the fans by the time the main event rolls out.
It is quite possible that WWE don't want any such thing to happen at SummerSlam and thus don't want to clutter the card with unnecessary matches. This would ensure the matches on the card can average decent times.
It can be argued that talented Superstars like The New Day and Shinsuke Nakamura are being kept off the card only to make way for part-timers like Trish Stratus and possibly Goldberg, It must be noted that major Superstars returning always helps in generating more fan interest.