Hush. Don't question WWE booking logic.
You know better than to do that.
Of course it's "best for business" for the guy who works limited dates each year to win the title over the guy who's been on top for 10 years, never leaves, shills the Network like a jackass and has everybody fed to him. Those broken arms are a distant memory. You're not even supposed to remember that stuff. You're a WWE fan. You're supposed to be an idiot that swallows everything without question.
All you need to know is that Brock Lesnar is Paul Heyman's client who broke The Undertaker's undefeated streak at WrestleMania. That's it. You're not even supposed to remember that Cena already beat Brock once before.