Some, albeit a few, of Comcast's systems have gone all digital. This needs to happen systemwide in order to free up bandwidth currently wasted by analog. Until then, they are probably limited in what they can carry.
At the same time, as previous poster noted, you should take the time to place a quick call and send an email to Comcast encouraging them to pick up the Voom HD offerings.
Cable has invested heavily to win back sat customers and in many ways have gained a technology lead. They have no choice but to match the offerings of dbs in the near future.
Call them, email them. Make some noise!