Why does unused cargo capacity of GVWR matter at all?

Those are the numbers for the trailers themselves, not your tow vehicle. Your tow vehicle has its own set of numbers for GVWR and GCWR that you must take into account when towing a specific trailer. As long as your tow vehicle is rated for the heavier trailer, then it would be fine for either trailer. If either trailer was loaded greater than it rating, then that trailer could be problematic.