It's because of vastly divergent views on ... well ... the point of Christianity.
The commenter queried, "Do we believe that the point of Christianity is to love one another, shun violence and hatred and care for the poor and needy. Or does it exist to get people to believe in Jesus so they won't go to hell?"
To be honest, if I had to choose, I would opt for the latter, though I don't think the alternatives are really quite that starkly opposed. Of course, I have no problem with love, non-violence, disavowal of hatred, and caring for the poor and needy. Those are important--yea, necessary--components of faithful Christian witness and ministry. But they are not themselves the "one thing needful", and I believe we are in error if we see them as "the point of Christianity." And while I would not choose to use language about the avoidance of hell to express "the point of Christianity" either, it is, I believe, less far from the mark.
Using affirmative terms, I would suggest that "the point of Christianity" is to make people fit to live in heaven, to be in the unfiltered presence of God without being vaporized by the sheer weight of divine glory. This is a process called sanctification (in the west; our eastern friends are apt to say theosis--deification). The process is fueled by grace, and grace, while generally ubiquitous, is found surely and certainly in the sacraments.
For my money, this is a lot more exciting than just trying to make the world a better place.