#3 Lack of believable opponents for Braun Strowman
Braun Strowman is an anomaly in the current roster in the sense that there are few people across the board that size up to him, to any degree whatsoever. As a current babyface or good guy, he'll want the heel to be a formidable opponent for the audience to be on his side.
This is why you really can't have him take on the likes of Andrade or even Nakamura, but there's a chance that the audience will begin cheering for the underdog. Samoa Joe would have been a great opponent for him, but he's currently in a feud with Roman Reigns.
Lesnar and Reigns are two other men who could face Strowman believably but they're both otherwise engaged at the moment. Force-fitting him into the card would have been odd.
This is probably why he was left off SummerSlam by Vince McMahon.