I apologize for adding to a load of already good answers, but towing capacity is often limited by tongue weight limits (how much weight is sitting on the tow hitch, and thus supported by the suspension), which should be 10-15% of the total payload. While it seems like they could just balance the trailer and reduce tongue weight, if it is below 10% the payload becomes extremely dynamically unmanageable.

The other limits come into play too -- torque, brakes, etc -- but tongue weight on suspension is often why there is a mismatch like this.