[cellarius]

Don't want to sound disheartening, but there are virtually hundreds of tech forums out there, that offer exactly what you offer and already have a big and thriving user base. What do you offer to your visitors that makes your forum special? The question is: Why should they want to register in the first place? Users of forums like yours either want to chat about your topics. Then they will likely join a more busy forum. Or they are looking for an answer to a question or want to ask a question. In both cases they very likely will start out by doing a search on google. They are a) not likely to find an answer at your forum, since there is not much content (yet), and b) won't come to your forum to ask a question when they have dozens of other forums they can go to.

As I said: I don't want to sound disheartening or harsh, but you asked why users don't join your forum. The question needs to be asked the other way round: Why should they? Answer to that question may leed to things you can do to make your forum more attractive. Most important: Content, content, content.