Earlier quoted context omitted.
Different buyer preferences. Different needs. Of course, it helps that most low-powered cars sold in the EU will have manual trans, where all US MBs are autos... Also, US buyers will run the AC a lot more, and AC eats a lot of power...potentially as much as 15-20HP... that's a lot more noticeable with a tiny engine.
Still I doubt if it is actually justified, considering in India a similar size car is perfectly serving its purpose with a 60 to 70 HP engine.
The US is a mature product.
High end brands don't sell sub-lowend products.