What are the more inherently secure distros and why?

And please include the BSD's in there as well. i do know they are not Linux. :-)

I'm also interested in why the most secure distros aren't more popular for security reasons? Or maybe I'll find out that the distros I hear about all the time ARE the most inherently secure. I realize there are trade offs with everything. What are the downsides to security? I'm thinking it has to do with the difficultly and time consumption?