You would think after hearing about how big Netflix is now, about how many viewers they get through original programming, and about the number of subscribers Netflix is gaining every quarter HBO would want to compete directly. Maybe you could argue that HBO does compete with their HBO GO feature which allows current HBO subscribers to access their content from a variety of devices. However to get an HBO subscription you need to have a cable package which has access to HBO. Despite having content people want and the technology to deliver it to them HBO has recently said they don't intend to sever the cable subscription ties just yet.
Recently HBO CEO Richard Plepler mentioned how HBO GO would also make sense packaged with a cable or phone internet connection, which gave some people hope. However Jeff Bewkes CEO of Time Warner, parent company to HBO, just mentioned that separating GO from cable wouldn't be in their economic interest right now. He also said that the US didn't have a substantial market for an HBO streaming service just yet.
I understand not wanting to endanger your ties with cable companies who are your main and only true source of income. That does make economic sense. But to say there isn't a market when Netflix has more subscribers through the internet than HBO has through those cable companies is seemingly naive. Maybe Bewkes is biding his time until he can feel secure with the HBO brand before pissing off cable companies but now seems to be the time to branch out into new media before old media all dies. So for now don't expect HBO GO anytime soon if you already don't have HBO.