The whole point of bringing out a new generation of hardware is to make it work in much better ways of operation than the last one. By default it is not going to run the older generation of games because it doesn’t work in the same way. Now they could spend a lot of effort in making it able to play the old games and work in the old way, but what is their incentive to do that, compared say in starting work on the next generation or releasing the console earlier?