15 tons for towed artillery is lighter than the same size self-propelled.
Towed artillery is *always* lighter, always able to be carried further by helicopter and cargo jet/turbo-prop aircraft than will self-propelled artillery.
Towed artillery will *always* require less maintenance than self-propelled artillery, and will always have more flexibility (e.g. pull up a different truck to tow if one truck breaks down).
Yup. And the bigger towed stuff is unusable on a modern battlefield. Simple fact.