How did you first find out about the next show z that you want to watch? Content distributors know that often some is introduced to their next show z while watching x on y. They want to make y one of your dedicated content discovery places.
For me, at least, it's usually from friends who mention that show z is awesome, completely disassociated from where that show appears. Why would I be loyal to HBO when I know that AMC has some great shows as well, as does Showtime. HBO doesn't have the monopoly/near-monopoly on good stuff that it would need for their version to be attractive. I'm far from a normal TV watcher, though.
