[SkyAtWork]

When the show is initially airing, HBO would rather you subscribe to the cable channel, since the cable company pays them much more than Netflix or Hulu would.

When the season ends, HBO would rather you buy the DVD / Bluray, since the sale of those discs pays them much more than Netflix or Hulu would.

It's certainly possible that one of the digital vendors could offer HBO enough money to change their mind there, but it's a fairly daunting check to write.