It is not that they cannot (which is part of it, for reasons 57 mentioned), but that they do not want to, becasue the TiVo heads probably feel that a box recording HD DVR would be too expensive for the small number of potential uisers of a system, where a direct cable/OTA recording DVR, which is relatively cheap to develop and build, has a much larger potential user base.
I don't get where Cablecard is a total failure, except perhaps fewer sets including CC slots. Sure, it is not a raging success, but things are getting better. Cable providers now have to use Cablecards in their boxes (although they don't have to field marry the card and box, and can still use a relatively closed 2-way system), and are warming to 3rd party Cablecard gear more.
True2Way next year will hopefully invigorate the 3rd party cable tuner market.