I think initially they were going to fast track a Man of Steel 2 and somewhere that was changed to BvsS.
In June 2013, it was announced that Snyder and Goyer had signed on for a MOS sequel. In July 2013, Snyder announced that the sequel would include Batman. The project evolved from there. There's been no announcement or evidence that WB scrapped a straight-up Superman sequel because of Man of Steel's box office performance or reception. That's just what some folks like to think, and hey, they're welcome to feel that way. This is technically a MOS sequel, after all, since it does continue that Superman's story and feature the same cast...
"... after
Man of Steel finished and we started talking about what would be in the next movie, I started subtly mentioning that it would be cool if he faced Batman... You're in a story meeting talking about, like, who should [Superman] fight if he fought this giant alien threat Zod who was basically his equal physically, from his planet, fighting on our turf... You know, who to fight next?... But I'm not gonna say at all that when I took the job to do
Man of Steel that I did it in a subversive way to get to Batman. I really believe that only after contemplating who could face [Superman] did Batman come into the picture."
Snyder, on how Batman came into the film
But really, enough of BvS in this thread, and who cares either way how the project came to be?