Dr. - i think once a month airings would not keep people engaged enough.
They do try to roll from one show to the next like True Blood starts shortly after Game of Thrones ends so they always try to have one show in rotation.
I've never watched all of their shows but enough of them and I've never cancelled between seasons. There is enough great programming to keep me subscribed.
Been watching more or less since Dream On in 90's....
Sopranos, The Wire, Big Love, Deadwood, Boardwalk Empire, true Detective
plus comedies - Curb your Enthusiasm, Larry Sanders Show, Sex and the City, Extras, Entourage, Veep and now Silicon Valley, etc.
plus docudrama miniseries like From The Earth to the Moon, John Adams, Band of Brothers, the Pacific
Looking forward to The Leftovers.
just so much quality TV. I can't imagine not having HBO...I hate network TV generally...the only broadcast network drama I've watched in the last 10+ years is LOST.