As far as the original question goes, I'd think it's the Monastery metaphor that may originally attract the notice of many Christians, and it's the content that keeps them here. But that's just a wild guess. ;)
Let's not forget other varieties of monasticism, such as with the various flavors of Buddhism.
One thing ironic about this site is that, due to the nature of the online forum, perlmonks is a lively social environment rather than an environment of isolation typical of most monastic traditions.