Durability may be a factor, but less capabilities must also play a major role.
Obviously they are smaller which saves weight, but specs show they also have smaller holding tanks, most don’t have slides, chassis are not designed to pull large trailers, etc. Engines are usually diesel but small 4 cylinder so that saves weight too.
Most Euro Class A and C are low to ground like vans so they don’t have much basement storage along sides, rather consolidating exterior storage into one large garage at rear. They are also not made to carry 1,000s of pounds of our stuff.
I personally think most older Americans are unlikely to ever want compact Euro-size campers, but younger buyers may in time. I’m just young at heart, ahead of my time.