My kneejerk reaction would be the higher rural/urban ratio, but I don't think that's right, becaues Russia has even more rural space.
It's probably more due to the fact that many of the roads in the USA were built a lot more recently, and were built to make it convenient to use larger tractor-trailers (and not so much for other road users). In Europe, truck operators had to fit into the existing road system.
The US Interstate system is extensive, well maintained, and fast. 60-80 mph will be the average speed for a truck on these roads.
If you are a long haul trucker you spend 95 percent of your time on these roads. You want high speed stability that a longer wheel base provides and don’t care too much about tight menuevering, because you probably don’t have too much tight area navigating to do. You will drop at a large warehouse on the outskirts of a city that was built for trucks like the one you are driving and be back on the road to the next warehouse.
America has large cities scattered across the entire country. When I look at a map of the cities in Russia, the majority are in the far west and south with huge amounts of the country without any large cities.
It's probably more due to the fact that many of the roads in the USA were built a lot more recently, and were built to make it convenient to use larger tractor-trailers (and not so much for other road users). In Europe, truck operators had to fit into the existing road system.