How about because it might be a good movie? I'm not a Christian, but I thought POTC was a great movie. Are Evangelicals only supposed to enjoy movies if they are Theologically Correct?
Of course evangelicals enjoy movies for entertainment purposes, but we're not (and I can only speak for those who are faithful to the Bible) going to "enjoy" a movie that is making a theological statement that runs counter to the very beliefs that make us evangelicals. Maybe there will be some kind of "novelty" entertainment value for many who reject the authenticity of Fatima but true Evangelicals are not going to support a film that is pure, unadulterated Romanism rather than a film that appeals to the broader spectrum of Christianity that we can all agree on.
POTC was different because we could come together around a film about the "passion" of the Christ. We agree on the historicity of the events although we differ on their interpretation. POTC didn't "force" us to accept one interpretation over another. Fatima is different because evangelicals don't accept it as a historical event, or if we do, we don't accept it as a true witness to the gospel of Christ and the word of God.
This will be a denominational film, nothing more. I'm certainly not arguing against Mel's right to make it. I'm just being realistic about the divisions between Protestantism and Romanism on issues like Fatima as well as a plethora of doctrinal differences. We really are diametrically opposed on many essential fronts (rather than non-essentials where unity is not required).