#2 They didn't have enough time to tell the story they wanted to
Perhaps the biggest handicap that WWE did with their scheduling this summer is the fact that they had less than a month between Extreme Rules and WWE SummerSlam 2019. Given that it was only a few weeks apart, WWE really wasn't able to play out storylines as they usually do for the summer.
Even the Roman Reigns angle, for example, happened only a couple of weeks out before SummerSlam. WWE knows that it wouldn't be wise to rush the story and have a Reigns vs Bryan or Reigns vs Buddy Murphy match right away.
While Roman Reigns would obviously add to the starpower of the card, the reality is that the timing wasn't right for them to pull the trigger on a match. Most likely, they felt they can give it the shine and spotlight it deserves after the biggest party of the summer is over.