Because the networks currently can't get paid for HDTV (and in fact, take away from their counted "paying" customers), so far the networks have done HDTV as a game of costly bragging rights, with only a couple of the largest cable networks jumping on board.

When they can actually charge advertisers for the HD customers as well (i.e. they "count"), cable networks, who couldn't afford to "lose" customers to their HD version, will begin exploding into existence.