#2 Outside interference
As much as surprise returns or attacks by returning Superstars shock the WWE Universe and get a lot of people talking, for a main event of this stature, and for the brand's main title, an outside interference would not add much value to the match.
Paul Heyman causing a distraction and then Lesnar taking advantage and the win, or Dolph Ziggler/Drew McIntyre coming to the ring to settle scores with Strowman, may do more harm than good for WWE, and Strowman, especially.
Fans drowned out a popular wrestler with 'She's a racist' chants recently
A clean finish to end the match will give Strowman and WWE momentum to move on to the next feud, and make the Universal title interesting again.
Perhaps the Wyatt Family could reform and make their presence felt following the match, which would make Strowman an even tougher Superstar to defend.
It's Strowman's time now, WWE!