I think a large reason for this was the FORCE that Christians used (and still use) to spread their religion. I mean, back in the day, if you didn't want to be a Christian, you were killed. Hell, I'd convert in that case. I know it's not the only religion to have used this tactic, but it was the most effective, for other reasons, I guess...
One of those reasons might possibly be that, as someone said somewhere in this forum, you have a lot more to lose if the Christians are right and you're not a Christian than you do if you're right and you're not a christian. If a christian and i died right now, and he was right, I'd be cast into eternal fire and whatnot. However, if I was right, he'd be right on level with me. Why NOT convert? Cover your bases? For me, it's a matter of principle is all.