From the little I know, the US considers it more polluting because of whatever unique emission profile. Europe thinks the better mileage more than negates that and in fact makes it better. After VW dieselgate, I think the US's view is considered correct. Not sure where it stands now with diesel exhaust fluid.
Also iirc from what I'm told, Diesel had a bad reputation after poor engines in the 60s, 70s or so. So on a personal level it wasn't popular in the 80s, 90s, etc. Car makers responded and didn't use them in cars - now it's pretty much only VW for diesel cars. Engines are better now but the trend carried even if the reason is forgotten. Big rigs and big ass trucks to haul RVs require it. You pretty much only see them when you require the torque.