People want to see it, because it's just another re-enactment of Christ's life or a part of it. Just like JCS was so big when it first came out in the theatres, even when it was a play it was huge. They want to see how someone else portrays His life, so they can either ridicule Mel, or give him praise. Just like people do with everything else.
The only group of people I can think of, wanting to go into a movie because of blood and gore is a group of 8 year old boys. But, you know.