#2 Brock Lesnar's reign would be far too short
We often complain that unlike say NJPW, a lot of WWE title reigns are far too short. For a title reign to be meaningful, it needs to be around the waist of a Champion for a long period of time. The fact that the RAW Women's Championship is so respected and revered at this point is because Ronda Rousey carried it for a very long period of time, and so has Becky Lynch.
Brock Lesnar just won the title at the premiere of SmackDown on FOX and for him to lose the title so very soon at Survivor Series would make it just far too meaningless in my opinion. If the match with Rey Mysterio were happening at WrestleMania instead where Rey Mysterio triumphed, it would have been far more meaningful. And the audience may not even be invested in Mysterio to the extent that WWE wants them to.
The art of telling a story long term may be lost in the age of social media and knee-jerk reactions but WWE still makes many old-school decisions. They would not want to water down Brock Lesnar's title reign with an extremely impulsive decision.