It's hard to see why. This is a pretty routine Costa-Gavras political thriller, not helped by unimpressive acting (even from Matthieu Kassovitz in the lead) and dialogue which was either dubbed or spoken in English with fake German accents. The lip-sync looked too good for dubbing to me.
The narrative concerns an SS Officer (played by Kassovitz) who finds it increasingly difficult to reconcile his increasing involvement in the holocaust with his Christian convictions. He forms a friendship with a minor official in the Berlin Vatican embassy and through him attempts to persuade the Pope to publicly reveal the existence of the Eastern European concentration camps. The Pope [the film implies] is reluctant to do so, ostensibly because he believes that by keeping a channel of communication open with the Nazis he can limit their damage, but in reality because he doesn't want to endanger himself or the Catholic hierachy.
I can't comment on the historical accuracy or otherwise of this portrayal, but the way it came across was totally unconvincing. The accents were phoney, the costumes and sets looked too shiny and squeaky-clean for a war-torn Berlin in 1943, and the characterisation was exaggerated to the point of being two-dimensional. You might find it hard to believe, but in the final climatic scene in which the Auschwitz commandant orders the death of a Catholic priest with the words 'Up ze chimney vith him!', people actually started laughing in the cinema, and I don't think this was because I was sitting in an auditorium full of Christian-hating Nazi sympathisers.
And as for Kassovitz, I think that anyone who has seen Les rivieres pourpres will agree that he should stay behind the camera.