Back in the day, BMW was the 'Ultimate Driving Machine' They put effort into being efficient with relatively low weight, relatively good aerodynamics and a pile of suspension tuning. They didn't necessarily have monster motors, or the best 0-60 times, they were more well rounded than that. It's that philosophy that I found attractive at the time.
CUVs have worse aerodynamics mostly due to larger frontal areas and underbody air management. Fastbacks aren't necessarily better than a squareback. It depends on how well the airflow is managed.
CUVs do have higher curb weights and tend to go with larger wheels to help balance the look of the side profile, so higher unsprung weight. To top it off, they have a higher centre of gravity.
Essentially, they're chasing the market, and putting form over function and efficiency. Ultimate Driving Machine was dropped a long time ago. They've added a bunch of models that perform pretty well for what they are, but they've really turned their backs on their old mission statement. It really comes off as a branding exercise now.