Ok, I'll give a detailed answer.
I just looked at your first post in that thread, and now let me explain
1. some people are just bored, they click everything, but don't bother to post
2. "contests threads" are often viewed by professional group of "contest whores", people who post in all kind of contest threads, but they prefer the "$50 for every 100 post" formula
3. with so many scammers here, you can expect some people dont trust any kind of contests at all
4. since you said you'll pick random winners, it can mean just that you'll chose your friends and send them an Ipod or whatever
5. speaking about Ipods, 60% of webmasters here don't live in America so they'd probably have to pay customs for a gift from the US
6. you didn't even say clearly what kind of prizes you'll be giving out
these are just a few possible reasons why not everyone reading your contest thread, posts in it