As others have said older Mercedes had very good reliability, so that probably still rubs off on how people feel about cars today. I'd consider BMW more sporty than luxury.
For instance, the base A-CLass (small/mid size sedan) sold in Germany has a 110hp engine and cloth seats, and very few standard features. Base price (w/o VAT) is ~$24,000US.
The base US A-Class has a 190HP engine, leather, more standard features, and starts at $32,500.
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.
The US is a mature product.
High end brands don't sell sub-lowend products.
That's been true for taxis in the US for a long time --- which are usually full-size Chevrolets and Fords. They're also much roomier and thus give a more comfortable ride than the average import econobox, with extra capacity for luggage.
There have been going down market in the U.S with the C class, then the CLA and now the A class, but these are still low-end luxury cars.
It also may be that because Mercedes and BMWs are "local brand" vehicles, there are more of them, and more mechanics out there able to service them? Whereas (to them) foreign cars like American vehicles you have to take to special American automobile repair places (like we have European auto repair places here that specialize in those makes/models)?
Again - guesses...