Not TV: Object Lessons in HBO's Sustained Brilliance is a profile of the company by Jack Lechner in Good Magazine. The article reveals much about the industrial constraints that produced the great string of successes beginning in the late 1990s that established the brand as we know it.
HBO moved more heavily into original programming when DVDs began replacing VHS tapes and when buying began to rival renting. Release windows were shrinking, so the company needed to give subscribers a reason to keep paying. In order to create the qualities that distinguish HBO from other offerings it gave creators more freedom than the networks. It wasn't as concerned with ratings because it's not selling ads, so prestige items that don't draw huge audiences would still be valuable for the perception they create that HBO is good.
I would still like to know how much of HBO's income is from DVD sales and rentals rather than subscriptions. This could be another point to connect the rise of HBO's original programming with a technological and cultural change.