The rumors I heard was that they wanted to call the film “Man of Steel 2: Dawn of Justice” but they pretty much had to go with Batman v Superman because that’s the name that all the hype has been around for so long. They had also been looking at other subtitles like “Knight Falls” etc. that would imply Batman, but they pretty much have to make sure Batman get prominent billing.
First, they have to make it clear that Batman is the antagonist here, WB and Snyder have said repeatedly that Batman and Superman will be fighting here, and still a lot of people seem to think that they’ll team up. I imagine they probably will team up, either against a larger villain revealed after they fight, or perhaps at the end as a lead in to the Justice League film, but they definitely WILL be fighting. We’re going to see them team up, let’s see them fight first.
And second, while the comic nerds and some of the fans more into the DC universe might get a clever name, the casual movie audience may not get a less obvious reference like “Knight Falls”. And sure, I’d love to have seen the film calmed “World’s Finest”, and it would give all the comic nerds stiffies, but that means absolutely nothing to the general movie-going audience, and also goes against the first issue of implying that they’re teaming up rather than fighting. So they pretty much have to have both Superman and Batman featured prominently in the title and since it’s been called Batman vs Superman, and it just doesn’t sound quite as good the other way around, I understand why they’re calling it that. Yes, I’d definitely have prefered to see a more clear-cut MoS sequel or at least a more poetic title, but ultimately what matters is that they get the butts in the seats and the film is good, the title ultimately is just a tool for the former.
That said, I wouldn’t necessarily say that this being now BvS officially means that we might not see a more direct sequel to MoS. Perhaps they wanted it to be more of a sequel, but as they got writing and started to develop more films, they realized they needed to give both heroes more equal billing to set up the follow-up Justice League film, then they’ll do Superman and Batman solo films following. After all, WB has officially announced they’re woring on 9 DC films currently, though several of them have nothing to do with this cinematic universe, so I’d like to think there’s longer term plans in place.
And for those who question how Batman can fight Superman, I like the comparison upthread that if Lex Luthor can give him a run for his money, why can’t someone who is just as smart, just as rich, but also a bad ass super hero, not give him a run for his money too? I would strongly recommend reading or watching the recent animated version of The Dark Knight Returns to see how Batman is able to fight Superman. There’s also an animated film from a few years ago called Justice League: Doom, where Batman has plans to take out any member of the Justice League should they go rogue, and they get turned against the Justice League. I haven’t seen Justice League: War yet, so I can’t comment on that one.
Admittedly, most of his plans against Superman revolve around Kryptonite, and that hasn’t been introduced into the universe yet, and I hope they leave it out. We saw that the Kryptonians in their battle armor were able to give Superman a run for his money in Man of Steel, so I see no reason why LexCorp and/or Wayne Enterprises can’t get their hands on the technology and Batman is able to build a battle suit out of it. Zod, while a general, was always at a disadvantage against Superman in Man of Steel, because he was caught off guard in their first encounter, and in the last, he wasn’t entering the fight prepared, he was in a straight up rage and fought recklessly with powers that, while he had gained some control over, obviously he didn’t have full control over (point taken from when he seems to accidentally use his heat vision in the building while still wearing his battle armor).
So Batman, an experienced hero with more than a decade of crime fighting under his belt and experience fighting villains that are more powerful than he is, could very well gain an upperhand by having a similar battlesuit, studying Superman extensively before engaging, entering the situation with a plan, and most of all being calm and calculating as he fights rather than enraged. As such, I think Batman will serve as an even more dangerous foe to Superman than Zod did.
TLDR: I’m a little disappointed in the title, but it makes sense based on where they’re going. There’s a ton of rich material to pull from where Batman and Superman can have a compelling fight despite Batman not having any super powers.