I think the site deserves more credit than just "Well, it just happened to be there". Internet forums and discussion boards have been around for quite awhile. Before Facebook groups, there was USENET and phpBBs, some of which still have scientific discussion. And of course, scientists have the ability to use Facebook and other massive social networks to have discussions.

But of all these, Reddit's structure and philosophy (how many other massive sites don't have a shitton of JavaScript applets and social buttons, besides HN) facilitates both the ability to easily participate and moderate...which seems to be integral for these communities to be as active as they are.