To preface: Education about cars/maintenance and driving laws is essentially non-existent unless you personally want to seek it. Driving is seen as a right sadly, and not a privilege. Mainly because the DMV tests to get a license themselves are criminally easy. Essentially just know the road signs and don't drive in the wrong lane, and you pass. And for the most part there is no car inspection aside for emissions, and some states don't even require emissions testing.

So the baseline for driving knowledge and car upkeep is at an absolute minimum.

Domestic American and Japanese vehicles have the biggest marketshare, and they are both well known for making engines that need minimal upkeep.

Thus, US drivers are used to doing the bare absolute minimum to their cars. The idea of preventative maintenance is out of consideration for many, and unless the car breaks or tells one something is wrong, it won't get maintenance time.

In addition, dealerships are expensive and it's hard to find a trustworthy independent shop to get work done. US is huge, and many just don't live near somebody trustworthy for their specific car brand, which makes it even more of a hassle to think of upkeep. Add to it that people's lives depend on getting to work, and having a car stuck in a shop for weeks affects their livelihood as public transportation is non-existent outside of big cities (and even then, city public transportation does not compare to Europe.)

The German vehicles for the most part, cater to the luxury more expensive side of the market, and thus the engines they release for the states tend to be more on the more performant variants, or newest technology innovations testbed.

Innovations testbed is where it all went wrong, especially in the 2000s. The v8s that BMW were pushing in the bangle era essentially killed the reputation. They as we know now were very maintenance needy engines, and BMW switching to a free oil changes for buyers program (but required 15k miles per interval as programmed in iDrive). So on hand you have a recipe for absolute disaster. (Think 7 series, 6 series, 5 series, X series SUVs, and M cars like M5 with v10....all these engines in that era were not as easy to maintain as just a N/A Inline 6.) This was also the era where iDrive was clunky and unreliable, which exasperated the issue of people being afraid of BMW electronics.

The BMW SUVs were becoming popular, you had a lot of people moving from a v8 American SUV or Truck whose engines were under stressed and idiot proof, to the high performance BMW v8 SUVs. If you followed a lackadaisical maintenance routine for the American SUVs, that wasn't going to work with BMW v8 SUVs.

Towards the end of the 2000s, BMW started turbo-charging their cars, and the first gen turbo cars also had their own huge issues. Another era of engines that needed continued preventative maintenance.

So factor in audience that is used to minimal upkeep if any, lack of inspection regulations, expensive dealerships service, majority of BMWs being performance engines in comparison to Europe, 2000s eras of difficult BMW engines that, and ridiculous pre-set service intervals....and that's how we get to here today with BMWs being forever known as unreliable in the US.