I think it's more a case of "like attracts like". Most groups will gravitate towards one type, so when you've got facebook type groups, a few gecko keepers will attract more gecko keepers. Fewer retic keepers will want to join a group that is mostly gecko keepers, while more gecko keepers will join, and you eventually end up with a group that is mostly gecko keepers.
With real life clubs, the same sort of thing happens. A few gecko lovers will encourage others in keeping geckos, and sooner or later the predominant species will be the majority of keepers.
Plus, geckos are cute, easy to house in a small area and relatively easy to care for. Girls don't mind a reptile who is also "cute", while guys tend to prefer a "cool" snake than a cute little gecko. It's hard to maintain your manly exterior when you're holding a tiny gecko.
The guys who keep geckos are unusually adorable though. Gotta love a guy with little lizards.
That didn't come out right.... I'm gonna shut up now.