Yes there is a good reason not to use nukes. And, in my opinion, that is because there would be no good reason to explain why they were used, after they were used. Meaning, the need to justify its use, after the fact, would sound lame, since we could not use the reasons that made its use in WWII valid, i.e., saving millions of lives on both sides if the war continued. The U.S. would appear like a Hollywood version of a trigger happy drunk cowboy, back from a cattle drive, in my opinion. A very bad image to give to a world that within a hundred years might be as advanced as the west. Or, how ye sow, so shall ye reap.
Your inference that they should be used, if I am not misconstruing your thread title, makes me wonder if you have some sort of impacted fecal matter, and need a good laxative to feel right with the world again? Just a little scatalogical humor, if you don't mind.