I do think fear is a big part of it.  The fear of failure, disappointment and the fear of being out of your comfort zone inhibits many people (me on occasion too :)).  The realization that God is bigger than our fears sounds easy, but is hard to practice.
I also think there is a bit of "I'm-better-than-you-are" &amp; blatant discrimination.  Moving towards faith and being invited to encompass more than just one idea is much more inviting and more freeing than one size fits all.