Special mention: Too many appearances in too little time
The whole charm of the Demon King character lies in the fact that it does not make an appearance regularly. Therefore, if you consider the fact that Balor was The Demon at WrestleMania against Lashley and then at Super ShowDown again, you know that having the Demon appear at SummerSlam would have been overkill.
Having the Demon show up is also generally a giveaway that Balor is going to win the match. It generally happens at the end of a long feud, like in the case with Bobby Lashley or Baron Corbin at SummerSlam last year.
Fans drowned out a popular wrestler with 'She's a racist' chants recently
While I can't be certain, this could have been a concern for WWE. You know that an overdose of something can ruin the fan experience.
And if you want an example for how that is the case, look no further than Roman Reigns and how the crowd rejected him.