i too would like to know this. not sure why some heroes & icons channel i've never heard of until last week that only airs garbage reruns is in HD and sundance HD content is still limited to on demand only. very frustrating having to wait a day after just to watch excellent series. pretty sure comcast is the only provider that doesn't have sundance in HD.
Comcast isn't the only provider lacking Sundance HD, but they're certainly one of the largest without it now. Dish and DirecTV have it. AT&T, Verizon and CenturyLink have it. Charter and Cox have the channel, as does most of Suddenlink. Only Cablevision (who oddly used to own the channel) is missing its HD feed among major providers.
As always, it's contractual with these things, and AMC Networks who owns Sundance either hasn't come to a new agreement to include Sundance HD yet (and sister BBC World News, which is also in HD). A new contract would bring Sundance HD to Comcast subscribers.
There's been some rumors about Comcast going to an all IP delivery system. If that happens, I suspect Sundance will launch in HD, as well as a lot of the other things Comcast subscribers are missing that might even be found on other providers' networks.