Why didn't they? A better question might be why should they? I know that if the first thing I am faced with is a registration prompt I just go directly to some other content that I know won't put up hurdles to get to the content. Anything that I must navigate through that isn't pleasing is just incentive to go elsewhere. Sometimes pop up ads are enough for me to chose to not reward the content with add attention, I just leave.

It would be interesting to see if when people go directly to the content if registration per unique IP went up. I understand the urge to get people involved in the content, and there are probably ways to do so that aren't off putting and don't involve finding the little lurk button.

Letting people consume Hubski for say an hour before being prompted to log in or register with a pop up would be great. An hour would show that people have at least nibbled the hook and it might be a great time to reel em in. Past the hour a prompt every 6 hours of time spent on the page might be good, not enough to harass, more of another chance to join the discussion.

I brought this up ages ago and it didn't seem to garner any interest at the time, but I know there are many people out there who look at first internet impressions a lot like I do.