It's because mpeg2 was the standard when the first HD boxes came out. Mpeg4 is slowly making inroads (the mpeg4 codec uses less bandwidth and provides better picture quality).
It started a few years back when NFL Sunday Ticket and NHL Centre Ice converted to it.
Now Sportsnet World and National Geographic are using mpeg4 too.
It's understandable why both the broadcaster and the cableco want to migrate to mpeg4. It sort of sucks for people with older boxes.
You're older Motorola boxes can't do mpeg 4
So far the dcx3200/3400, the gateways, and some of the PACE models can do mpeg4.