Except all of those characters except Black Panther were in previous films and this conflict has been boiling for like four years.
No it hasn't.
And it all boils down to good writing/film-making. When I look at the trailer, the biggest problem I have with BvS is that it looks like they're trying to shoehorn too much into one movie. You could certainly say that Marvel already has the established characters to build on, but we don't need that with Superman and Batman...it's fucking
SUPERMAN AND BATMAN.

And this is a sequel to that last Superman movie anyway.
Ditch the Justice League stuff or just make a full-on Justice League movie instead, or take the Marvel approach and (re)introduce the characters in their own movies...then bring them together. It makes more sense.
One thing I think that DC-based live action stuff has over Marvel are the TV shows. I've been watching Arrow and The Flash and they are both awesome. The whole "arrowverse" thing has done an amazing job of establishing characters and bringing super-powered characters to life as well. The shows are way more exciting compared to Agents Of SHIELD. And yet Warner Bros. is NOT using these actors on the big screen and they're not linking all their shows together. Why is Supergirl not part of the arrowverse? I can understand Gotham not being part of it, at least.