As emissions and efficiency standards continue to increase, does the need for new technology. That is the reason engines don't carry over from model to model. An engine introduced in model year 2000 can not meet todays CARB or CAFE standards, let alone both. Not to mention as they age, the issues start to surface. Take the secondary air passage issues with the S62 as an example.
The easiest way to keep up with the horsepower war that the public demands while keeping the engines from growing in size, is forced induction. Engines can only make so much power while maintaining their longevity.