Reddit didn't get a lot of press attention because -- among other things -- the people behind it were not easy to find and talk with. (They still aren't.) If as a journalist I can't connect with you to, say, fact check something I'm writing, your chances of being covered are much lower.
Plus, when Reddit was on the upswing, there were a _lot_ of competing sites that were trying to be "Digg for [niche]," such as DZone as "digg for developers" and dotnetkicks as "digg for the .net people so we can be even more granular," and agileblend and so on. Not to mention older sharing sites like delicious (which is, um, still around?). So initially it seemed like "yet another link sharing site," and one that was far more confusing even to us tech press geeks.