The short answer is that the US has not signed on to the United Nations Economic Commission for Europe standards for automobile design so the emissions and crash testing data are not 1:1 comparable.
The longer reason is that grey market importing gained popularity in the 80s which significantly ate into the domestic market (including dealerships) for Mercedes Benz in particular. Mercedes Benz launched a multi million dollar lobbying campaign to stop the import of grey market cars which was successful.
As a result of being practically banned, the grey market declined from 66,900 vehicles in 1985 to 300 vehicles in 1995. It is no longer possible to import a non-U.S. vehicle into the United States as a personal import, with four exceptions, none of which permits Americans to buy recent vehicles not officially available in the United States.
In 1998 the 25 year rule was implemented because presumably a 25 year old car would be considered collectible.
In 1998, NHTSA granted vehicles over 25 years of age dispensation from the rules it administers, since these are presumed to be collector vehicles