#3 Vince won't bury his guy
There have been numerous instances in WWE of people from other industries stepping inside the squared circle to challenge WWE Superstars, and though few them have come out victorious, it was not against one of the top guys in the company.
Even someone like Ronda Rousey was not allowed to go over the home-star Charlotte at last year's Survivor Series showdown between the two. While someone like Braun Strowman might once be considered to put over Tyson Fury, it's very tough to imagine Vince McMahon allowing Brock to do the same for Cain Velasquez.
Brock Lesnar has been one of the top guys in the company ever since he broke the famous streak of the Undertaker in 2014. Currently, in his 8th World title reign, Vince has allowed him to go over many of the top Superstars such as John Cena and Roman Reigns, to name a few.
A victory for Cain would mean that Brock's title reign would end in less than 30 days and would also see the veteran being completely dominated by someone who's still an outsider to the pro-wrestling community. Do you think Vince would bury his guy for the sake of a former UFC Heavyweight Champion? There would still have been some chance of him doing it, but there's one thing that would not let him do that. Which is...