The Miz headlined one of the highest bought PPVs in the history of the company as WWE Champion. Brock is still a special attraction without the title and any of the four guys he splatted in the Chamber would have been made to look a million times stronger by pulling off the upset and walking into Mania as champion. Why bother sacrificing EVERYONE to keep building someone who has already maximized his drawing power?
There’s plenty of arguing that having Brock Lesnar destroy absolutely everyone like he’s swatting flies is supposed to help sell WrestleMania. But doesn’t the show sell itself?
Because that's how they've always done it and that makes it OK, apparently.