Cargo bikes work better for heavier loads, most trailers seem to be ~30 kg whereas cargo bikes do 100-200 kg. And I wouldn't want to have that much on a trailer because of what happens when braking when towing a brakeless load on a wobbly arm that's heavier than the tractor.

Trailers also bounce around a lot when empty at any speed and can fall over if you're eg crossing a curb while cornering.