Someone, sometime ago decided that a vehicle above 3500kg might have a different behaviour on the road than those under so they decided that to manage a "heavy"

Someone, sometime ago decided that a vehicle above 3500kg might have a different behaviour on the road than those under so they decided that to manage a "heavy" vehicle, you had to have special training and a special license. In Belgium, they decided that this limit is the same with or without a trailer so a standard car of 1600kg cannot pull a 2000kg trailer. And someone with a standard license cannot drive a car that weights more than 3500kg. And if you have a car that weights 3100kg, add the driver and passengers, that doesn't leave much room for a trailer. And that's also why there aren't many fifth wheel trailers in Europe.