Because backwards compatibility allows for several things for those with a comfortable, but not overly so, financial situation:
With backwards compatibility, I can trade in an old system to try and offset the cost of the new one, while still maintaining my library of games.
If I decided to go with a different companies console, one in which I never had the previous generation, I can experience games I never got the chance to. Maybe these games end up as download, but maybe not.
Old games are a great way to have fun on a budget. I still, probably buy as many gamecube an xbox games as I do 360 ones, because even if used games are offered at pretty absurd price points sometimes, they are still less than a retail game.
With backwards compatibility, I can trade in an old system to try and offset the cost of the new one, while still maintaining my library of games.
If I decided to go with a different companies console, one in which I never had the previous generation, I can experience games I never got the chance to. Maybe these games end up as download, but maybe not.
Old games are a great way to have fun on a budget. I still, probably buy as many gamecube an xbox games as I do 360 ones, because even if used games are offered at pretty absurd price points sometimes, they are still less than a retail game.