Lol, because the other religions weren't as good at recruiting!
In all seriousness, I would have to say(I was brought up Southern Baptist, ack)that people want absolutes, certainty. They want"do this, and get to Heaven". They need the special feeling that abiding by a certain set of rules gives them, when they are with people of the same genre. Almost all of the "old" religions preach tolerance and incorporate several points of view or "possibilities" to their beliefs. As someone that does believe that Christ is the Saviour, but is not sure about the rest of it, that's my 2 cents.