Speed limits. Americans generally refuse to slow down when towing. In most of western Europe, the speed limit is 20mph lower (give or take) when towing. In order to tow at a high speeds while maintaining stability, you need a long wheelbase and mass.
Liability and fear of lawsuits. My 2017 VW Sportwagen is rated to tow 2000lbs in the EU. It isn't rated to tow AT ALL in the US. There are no mechanical differences that would impact the tow rating. If a consumer wants a vehicle rated to tow, they literally can't use a mid-size sedan or wagon in the US (despite the exact same vehicle being able to tow a small travel trailer, horse, or small powerboat without problem).