I believe its something to do with older boxes can't decrypt the new encryption method - and I think this even includes very early HD boxes which is why Sky had to upgrade anyone still on one for free a few years ago. They could have potentially had paying HD customers unable to access the content so they had to give them out for free. It also means all standard Sky boxes (none-HD ones) also are unable to deal with the new encryption, and I'm guessing there must still be an awful lot of them out there - enough so for Sky to deem it too expensive to upgrade them to a new box for free. Which is why the SD broadcasts stayed on the old system so that they can still be accessed. It will happen one day, it's just a case of when Sky deem it feasible.