You really should have done a little googling...
As of 9/2014, Roku had sold 10M units and Apple has an installed base of 20M and had $1B in revenues from it. So if only one was going to get it, Apple was the right choice.
Here's a nice article about it:
http://variety.com/2014/digital/new...-roku-apple-tv-fire-tv-chromecast-1201303129/
If we assume the "variety" numbers are correct...
http://variety.com/2014/digital/new...ion-apple-tv-set-tops-to-date-ceo-1201162424/
The purple box served an aggregate 37 million hours of video streamed per week compared to Apple TV at 15 million hours, Chromecast at 12 million hours and Amazon Fire TV at six million hours.
There is a big difference between global and US sales numbers. If 15 million of those Apples were in Europe and Asia and the service is US only like Sling TV's new service, those units are effectively worthless for HBO. Apple TV has been around for quite awhile in one form or another. Many of the early units were dumpstered when people found out they were pretty worthless. There was even a conversion to Boxee for the early Apples for because of the lack of content. The old ones are not being used so if they are part of Apple's 20 million number it is inflating the installed base.
Variety's numbers for hours streamed shows Roku at over twice the number as Apple. That seems to support that Roku owners actually use the product. They are the ones most likely to buy the HBO OTT service.
If you throw in the ever increasing popularity of Fire TV and Chromecast, Apple really is on the losing end of the numbers game. And we're not even talking about the millions of low cost Android devices in use such as tablets with HDMI that are very easy to hook to TV and take on the go.
There also seems to be the idea that Apple owners are the more affluent types. I'd venture a guess that many of this type also have their cable tv or satellite and are going to keep it. Many "affluent" Apple owners also happen to be technology dumb and aren't as likely to do something that requires thought, research and work such as putting together a real cord cutting package on their own. HBO or Netflix are just one part in doing it.
It is fine by me if HBO wants to limit itself to Apple. It saves me $15 a month and is one less thing I have to support on the A3's or help my less affluent friends and family with Android and Roku to make work.