I had a conversation with a friend last night about religion, and he asked me why it is that the prominent religions seem to be based on the idea of wiping out any other religions.
We can't ever know for sure, but natural selection certainly seems like a plausible reason. Richard Dawkins has suggested that memes follow a similar pattern to genes in this respect.
When one religion preaches passivity and tells its followers not to evangelize it will eventually die out as its followers are killed by more aggressive religions. Its culture is soon wiped out in favor of the religion "designed" with multiplication in mind- specifically, the religion that says to kill the infidels and to convert unbelievers.
It helps, of course, if your holy text can be interpreted to say both. That way you can multiply at the expense of others while still managing to avoid being perceived as a threat.