and if I remember correctly, it became hyper apparent to the Army that in the case of an attack in Europe, our biggest artillery guns (at the time) could only fire 13-14 miles
The blast radius of the 15kt nuke was something close to 3 miles it would have taken more time to call in the attack and get Prez permission to unlock the nuke, take it out of its secure location, arm it, load it, and fire it than it would have taken for the soviets to get within 3 miles or less of the gun (probably even would have overran or bombed them to Bolivia).
And of course they would have only gotten permission to deploy it in either Soviet Bloc territory, or uninhabited forests/open areas, and most importantly, only if the wind was blowing towards the same direction of the shell being fired .
rendering the idea of a nuke artillery shell completely useless. Hence the reason why this was the only one ever fired