1. Batman has been tremendously popular and successful, well before The Dark Knight trilogy. Due to having great comics, a great rogues gallary, the 60's television show, the Burton films, the super popular animated series, and his games. TDK trilogy only added to the popularity, but it didn't create it nor does it define it, just supports it.
2. Batman characetrs have mainstream audience appeal. Walk down the street, and ask 10 people to name as many Batman characters as they can. They'll be able to name at least 5 guaranteed. Now ask them to name at least 5 Aquaman, Flash, Wonder Woman, or Martian Manhutner, Green Arrow, or Green Lantern characters. That's the point. Batman characters have huge fanbases. Do you see any Aquaman, Flash, Wonder Woman etc. villains or characters beyond themself, getting ANY merchandise outside of a comicbook store? No. Yet I have seen Batman characters get merchandise at wal-mart, target, toys R us etc.
3. Batman fucking sells. How hard is that to understand? Put your bias aside and look at it from a business standpoint. Do you want this game to do well? Do you want it to appeal to the most people possible, gain new fans, and make as much money as possible? If you do, then what better way than to offer characters that appeal to the casual audience? The ones who may have only seen the Batman movies or cartoons, but don't know the other characters or care for them beyond the main Justice League heroes. Once you get their interest, then they can become fans of the rest. Stop making this a dick measuring contest saying Batman can't have more than a couple of characters. They are ALL a part of the DCUniverse. Accept it.
4.People knew who Bane was, before TDKR. Comic book fans knew who he was, and casuals likely saw him in the movie Batman and Robin. As bad as the movie might have been, it still sold plenty of tickets. Bane has also been featured in Batman cartoons, Batman games, Batman documentaries, and the animated film Justice League: Doom.
I also find it extremely hypocritical of you to even hate on Batman so much. I have seen your DLC wishlist, you know, the 18 characters you hope to get, and it included John Stewart, Atrocitus. and Black Hand. That would make SEVEN Green Lantern characters(including Grundy), with 4 of them being Lanterns. Do we really need 4 Lanterns? No. But you sure as hell let your love for Green Lantern dictate so. But 6 Batman characters? Oh man, that's outrageous! How dare the most popular, iconic, and recognizable franchise DC owns get 6 characters!