That's the major difference; time and freedom. On a regular broadcast network if a show doesn't perform well immediately, no matter how good it is, the axe can fall very quickly. There's little reasonable analysis that says, "we really have some thing good here, so let's try to cultivate an audience
." Instead they say, "Well, after 6 episodes we haven't broken this certain arbitrary number within this certain arbitrary age group, so we'll get rid of the show (and the 8 other shows that aren't working either) and try something new next season." Great shows like Sports Night and Arrested Development get cancelled after only a couple seasons, despite acclaim, because the networks haven't figured out how to get people to watch. The traditional networks just throw things at the wall until something sticks.
So, why is it that HBO has the luxury other networks don't? Well, the most obvious reason is that it's a paid subscription based network. What this means is that they don't care about ratings in the same sense as everyone else. They just care about the total number of subscribers, and as long as that number stays high, they can focus on creating quality television with interesting characters and bold subject matter. This system also means they are free from censorship, which can add a level of realism to their programming.
But even without this benefit, the traditional networks could still learn something from HBO. They could still take a bite out of their philosophy and simply try to make a better product. Personally, I can predict what shows will become hits and which will get cancelled after 13 episodes just by watching the first promo. So many of the new shows that come out are just horrible ideas with bad actors and terrible writing. It's no wonder they can't get viewers by trying to sell sell them bad ideas.
Ultimately, people want shows like Entourage. People want to see a show where over time the characters change and develop, and where the plot gets more compelling and complicated over time. In only the first 3 episodes of season 6 we can already see Entourage heading in a new direction again. All the characters are starting to get tired of being propped up by Vince and want to make something of themselves. Turtle is going to go back to school to learn business, Eric is developing his management group, and Drama is enjoying success on his fictional TV show Five Towns after years of working in obscurity. How easy would it have been for them to just stick with what works and parade the characters through an endless line of parties?