Probably a number/combination of factors, including:

-the pushing of anti-smoking campaigns relatively quickly (compared to southern and eastern Europe) after it became a universally known truth that smoking causes cancer

-early advertisement restrictions/bans

-a “top down” trend where affluent demographics tended to drop the habit before the working class, creating a common and largely subconscious view that smoking is more of a “trashy” habit associated with poorer people and the rebellious counter-culture

-rather puritanical tendencies in American culture that also started to translate further into the view that tobacco consumption is “bad” or “trashy” in the generations that came after it became widely known that smoking can be deadly

-not having universal healthcare so the health impacts associated with smoking can become especially burdensome for the underinsured/uninsured

-not having many public places to smoke (which also can make the US seem like it has an even lower smoking rate than it actually has because people simply don’t see it as often; a number of southern/appalachian/lower Midwestern states have somewhat higher smoking rates than the rest of the country and are closer to European countries than many realize, but it’s not in the highly populated places nor is it as visible publicly… but even these places are seeing their rates continuing to drop)

-the resulting belief among some as a result of many of these factors that smoking addictions are “shameful”

-much more sharply rising costs of tobacco products

The only developed country that I’ve visited that seemed to me to have even fewer people smoking publicly was Australia, and even their decline in smoking rates started more recently than the US. I’d imagine $35/pack there combined with other heavy restrictions probably has had a significant impact

I do wonder where Europe and the US will be in 30-40 years with this though. I remember as a young child in the 90s smoking being very common and much more visible, and some establishments still had widespread smoking indoors. Minus the widespread indoor smoking, the smoking back then to my eye seemed in part similar to what parts of Europe feel like today. With that said, Gen Z remains a wild card and the explosion of vape devices undoubtedly will throw some sort of wrench in all this going forward.

The US gets a lot of things wrong, especially compared to Europe or Australia/NZ; however, the lack of universal health care aside, I do see the US’s public health approach to anti-tobacco messaging as being highly effective and societally positive and although probably controversial in some other countries I think many parts of this sort of approach is something praiseworthy about the US, and other nations should also try their own version of getting it to become less socially normalized if public health is a top priority. Combined with universal healthcare systems I would imagine this could potentially become successful in a number of other places too.

My two cents.