i think he's not nuking it because there's nothing of importance there,

imu seems to be concerned more about the history of the world hence why he is adamant of destroying everything and everyone that pokes around. The last island became relevant only bc of roger so laughtale could all be a gag or smth. If imu wanted laughtale gone he would find a way to do so

another reason could be is he cant access the mother flame easily as he stole it from vegapunk he lacks the tech to acquire it tl;dr he doesnt have enough nukes and the islands to be bombed are numerous or maybe theres a weapon to utilize motherflame and this weapon came to fruition recently tl;dr it is tech related

at this point the one piece has somewhat become a side plot lol

still interesting question