The main difference between the two ideas that I can see is that Cena and Bray had history together, their whole run up to Mania 30, what Bray winning would have done for his career, the 'Cena buries everyone!' accusations... as it was, he lost and that started being when Bray started becoming known for losing all the big matches. I think I'm right in saying that's when the creepy cult leader started doing all the supernatural stuff, with the kid singing to Cena and all that and it's easy to point at this particular run as the moment Bray jumped the shark, and within three years, we've got maggots on the ring canvas. I still thank various deities that match between Balor's Demon and 'Sister Abigail' never actually happened.
Whereas with Strowman, he just appeared as a member of the Family and then left in the Draft when he got put on Raw and Bray was on Smackdown. There was no big split - teased or actual - so for the Swamp match, they tried to create all this tension and history between the two that simply wasn't there.
I've been thinking about this for a couple of days and this may be the wrong thread, but it occurs to me that Bray is the wrestling version of Stephen King. They both come up with some pretty cool concepts, when Bray introduces a new character, it's awesome (see: Fiend, cult leader, Firefly Funhouse) but then he's got no idea how to carry them on. I've read a lot of Stephen King books and quite a few times there's a sense he has no idea how to finish the story so he shrugs and writes in aliens or a supernatural entity. It's like that with Bray, the Fiend was amazing when he faced Finn Balor, then jumped off a cliff vs Rollins; the cult leader was amazing in his rocking chair then went to pieces vs Cena... the best, most intriguing character he has for me is the kids TV host, purely because he's hardly ever seen actually wrestling.