Hmm, let me look this up on Wikipedi......oh.
I think it has to do with length regulations. Generally, European trucks have to navigate smaller roads so having a shorter front end makes life easier for the drivers.
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You do see US-style trucks over here occasionally (mostly Scanias). I think its more to do with fitting the environment rather than a total length restriction.
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To my knowledge the shorter nose makes it easier to move through tighter roads and not have to worry about hitting things. The trucks also take up less space and are thus easier to park which is a huge concern in many urban european cities in Germany, Italy, France, etc, where parking is limited
The longer wheelbase of a long-nose tractor also makes for a smoother ride than a cab-over which is important if you're driving it 8+ hrs a day.
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Some parts of the US had similar restrictions in the 1960s and 1970s, so cabovers were common in those decades, but quickly lost popularity after length laws were standardized.
its a combination of most things listed above, and also some other factors:
such as the roads being very tight, more like highways instead of interstates like here in the US. the shorter rig lenght of the european model makes it much easier to move around, while in the US the nose is actually much more efficient at high speeds, like on interstates.