There are many factors considered by the vehicle manufacturer when setting a trailer tow weight. In my case I tow with a 1500
Dodge Ram with a 360 and auto trans and a 3:92 rear axle ratio. This same package in a 2500 is rated to tow a heavier trailer. The engine, trans, and cooling package is the same but the 2500 has a heavier rear axle with a 4:10 ratio, larger brakes front and rear, heavier frame rails and heavier springs.
In the case of passanger cars and many mini vans they are unibody construction and do not have a seperate frame rail to attach a hitch too. As a result the manufacturer usually limits the towing capacity based on the vehicle structure. There are companys making hitches who will build anything you want and not consider the vehicle structure.
And last but not least all the truck companys do handling, stoping and relaunch tests which also may reduce the approved trailer weight. The truck companys also perform structural durability testing under very severe road conditions.