#1 Why he should not retain: He is a part-timer
Even though Wrestlemania is just a few months away to happen, there is still a lot of time left for it. Lesnar is a part-timer and there's no reason why the fans don't like him as a champion. He has not defended his championship as a "real" champion and it's definitely making the fans go angry.
Still, he is champion and not for just a few weeks but for over 562 days (at the time of writing). Part-time wrestlers are definitely a problem for the WWE and it should be solved as soon as possible.
Strowman vs Lesnar is going to happen at Royal Rumble and it is the only chance for the WWE to take the belt away from him before Wrestlemania. As we all know he only works on big pay-per-views.
So maybe Strowman is going to defeat Brock Lesnar.