Two things:
Scale -- HBO is significantly larger when it comes to production capacity. HBO can also have far more shows in production at one time. That scale is partially a product of capital, but also one of point number 2:
Experience -- This one can't be underestimated. HBO first started toying around with original content and series in the early 1990s but it wasn't until Larry Sanders that the show really started to resonate beyond the periphery. Even then, the established TV world didn't start to pay serious attention to HBO until the late 1990s, when you had a swath of hits become bonafide cultural phenomenons (Sex and the City and The Sopranos were hits of a massive global scale beyond anything that cable, let alone premium cable, had seen up to that point).
Netflix has had to start from ground zero. They are essentially right now where HBO was in 1997, when HBO started to shift from acquiring content to paying for original content (Larry Sanders) to making the content in-house (Oz).
Now, granted the current world moves faster -- and Netflix is already potentially approaching the 1997 spot in just two and a half years with this strategy -- BUT content is also now more expensive (The Sopranos and Sex and the City were not $100m shows for a total of 26 episodes like House of Cards is), competition is fiercer than ever and the overall economics of television are not as advantageous as they once were.
I would also say that a third advantage is cable. Yes, cable is an advantage. Why? Because cable often serves as free (or nearly free) advertisements for HBO shows and the bundling of HBO for 3 months free or half-off or whatever is a huge driver in subscribers. Now, Netflix can replicate this, but they'll need to do it at a price and obviously they'll need to assess if giving the cable companies 30% of their monthly revenue is worth the advantages of being bundled.
None of this is insurmountable, but just as starting a movie studio from scratch is hard -- pivoting from a pure distributor into an original content creator/distributor is hard too.