Brock Lesnar has been the company's go-to guy for a number of years. This has never been more apparent than when Roman Reigns was forced out on hiatus last fall and Lesnar was then the man that WWE called in to take the Universal Championship and keep his seat warm until he returned.
Lesnar has been bouncing around the Championship picture in WWE since 2012, with the only real break coming when he and The Undertaker went to war throughout 2015. Lesnar has only recently dropped the Universal Championship to Seth Rollins, and if he moves over to SmackDown Live and is able to defeat Kofi Kingston, then he will be swapping one title for another.
WWE doesn't want to look so transparent that the only person that they can put the title on following this huge switch is Brock Lesnar. WWE is running out of ideas for The Beast, this is why it would make more sense for Kingston to retain and it will then be proved once again that Lesnar can be defeated, much like Rollins proved back at WrestleMania and SummerSlam.