Well, only 1% of the software "is Christian". The other 99% has already been created in various forms. I'm not criticising the project, I just don't know why he didn't extend existing software, or even fork something like BuddyPress.
Sometimes it's just fun to build something from scratch and make it awesome.
Also, if I had built OneBody using PHP I certainly would not have lasted this long in keeping it going. Ruby is great language that I really enjoy using every day.