Why Kilmer in the movie poster?

Kilmer was getting a lot of hype at the time and was fronting a lot of movies that were meant to be huge blockbusters. He never quite became the a-lister he was being set up as, though. A lot of those Kilmer-led movies ended up being duds.

Popularity and billing rights most likely. Val Kilmer was a big name in the 90's (easily an A list actor). I am sure he wanted top billing alongside the 2 main stars. Probably something he asked for in the contract. Or maybe the studio just wanted to capitalize on his popularity in that era so they throw his name on the poster and gave him top billing.

Not a big deal, it was just the poster. I think nowadays it is worse where they will actually start changing the story for the actor if the actor is a big enough name. This basically is creatively bankrupt.

As others have already mentioned more or less, Kilmer was at the height of his popularity/marketability at this point in his career so it makes complete sense that he would be prominently featured on the poster. Yes the theme of the film was definitely De Niro vs Pacino but Kilmer was still the third most featured character so it fits.

Kilmer was a boss in this movie.

You have to remember that this was almost 20 years before The Spoils of Babylon.

Val Kilmer was a big deal back then.

Val Kilmer was hot shit then. his career really peaked with this and batman.

Keanu Reeves was originally signed to play Chris Shiherlis, and Carsten Norgaard was also one of Michael Mann's options, but they both lost the part when Val Kilmer was able to squeeze it into his schedule while making Batman Forever (1995).