Ask HN: Why don't people share our viral page with their friends?

I'm a web developer. I love dev and I think that I'm pretty decent at it. I am definitely not a marketer, which is why I bring this question to you all. My wife and I are starting a fun side project together called Happy Nerd Shirts where we will sell shirts with designs specifically targeted at developers, blah blah... I'm not here to make a pitch, I just need your advice.

Since we're trying to follow in the footsteps of the startup greats, we've made a Launchrock page at signup.happynerdshirts.com where we are offering what we thought was a good incentive. We have it set up where their incentive increases in value as they get more friends signed up. The problem is, people are signing up but they really aren't sharing their viral links out with anyone. Is this because our infographic isn't good, is it because our incentive sucks, or is it because people want our products only for themselves? I'm at a loss, so I'd really appreciate your insight. Thank you guys!