#2 To protect Braun Strowman
Braun Strowman is almost guaranteed to be the Universal Champion someday soon. But like I've discussed on the previous page, it was probably not going to happen at the Royal Rumble. This would have essentially meant that Strowman would have suffered a loss at the hands of Brock Lesnar again.
Strowman has lost twice to Brock Lesnar already. He could not even get the job done inside Hell in a Cell against Roman Reigns, thanks to Brock Lesnar. Having him suffer a defeat at Brock Lesnar's hands would have completely killed the aura of Braun Strowman for good.
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Therefore, I think that WWE wanted to preserve Braun Strowman's mystique by cancelling the match and have Strowman win on another occasion. By having him lose again, WWE would have killed his credibility forever.