It's pretty obvious that from the moment "21-1" became a reality, the long term plan was for Lesnar to splatter Bryan at Summerslam* and run roughshod over the main event scene until Roman Reigns stepped up to slay the beast. A fine plan on paper that was scuppered in part by circumstances beyond creative's control. One thing they could control, however, was who held the championship between MITB and Summerslam. To that end, do you think they might have managed to preserve Brock's post-Wrestlemania-30 dream-smashing-mercenary-shithead heel heat if they'd kept Cena away from him for the most part? Obviously that would mean putting the belt on somebody else for the short term, but since "short term" is the operative concept here, why not roll the bones with someone a bit less "polarizing?" (Someone who might even gain a little traction in the process and plant the seed for a proper run down the road.) The Summerslam match was, ideally, supposed to be a shit-smeared iron boot on the fans' collective throat. Per Cena's participation, it wound up being the most cathartic televised murder since Joffrey's. That alone was enough to secure Brock conquering hero status, and the fact that the feud lurched on in slightly modified Cenaesque tit-for-tat fashion didn't help matters in the slightest.
*P.S. I can't help but wonder what manner of nuclear-grade heeling Bryan's absence from the Summerslam match deprived us of. Picture this scenario: but for an aborted hope spot or two, Brock obliterates Bryan in the same way he did Cena. He's got him dead to rights, just toying with him, hoists him up for an unnecessary F5 (to the tune of Heyman crying "YES! YES! YES!" at the top of his lungs), when suddenly Brie Bella hits the ring. Amidst acknowledgements from the announcers that she's Bryan's real life wife, she attempts to throw the towel in on his behalf...only to be prevented from doing so by Heyman (or somebody from the Authority, doesn't really matter who), whereupon she is forced to watch helplessly while Brock hits SEVERAL successive unnecessary F5s (to the tune of Heyman screaming "TIL DEATH DO US PART!" until his face is on the verge of erupting) before finally pinning him. Done correctly, they would have needed to escort Heyman and Lesnar out of the arena in a bulletproof car that night.
That's quite the PS. And yeah, I think everyone was totally fine with Daniel Bryan getting splattered in a ***** match because it would have legitimately given him the superstar rub from Brock and probably been the greatest David v. Goliath match ever.
That being said, for business Cena was the only choice they had. In a perfect fantasy booking world, sure, you put the title on Dolph Ziggler and let him get ragdolled for our entertainment, but they were paying Brock a shitload of money for those matches and squashing Ziggler wasn't going to make it back. Probably Daniel Bryan wasn't going to make it back either, but that was at least what they had previously decided. I think in a perfect world CM Punk is still around and they go with him as the interim champion instead, but I can only imagine the temper tantrum if they asked him to do another job to Brock. So really, it kind of had to be Cena.