The hold-up? Nothing, really. Comcast says the delay was due to product and platform evaluations. As for what that really means — Roku and HBO GO have both been around for quite some time; and the HBO GO app via Roku has as well — is anyone’s guess. The real reason likely revolves around Comcast not allowing access to anything that directly connects to the TV. You could source Comcast as your provider via apps, but until the Chromecast came along, that was a second-screen experience for most.
Comcast is still in the process of “evaluation” on other devices and platforms, most notably the Fire TV and PlayStation 3. If Roku and HBO GO are any indication, it will likely take some time before either platform gets HBO GO via Comcast.
