Use of nukes is punished with annihilation. That's the unwritten rule. Since it's unwritten, a lot of the fine print is unwritten too, like how soon and by what mechanism. Maybe, if you nuke your 150m radius it sets in motion a chain of events that five years later culminates in nuclear war. You have no way to know whether you would be opening the door to that. Since the loss is effectively infinite and the probability is finite, the expected value of this action is infinite and negative.

People want certainty when it comes to nuclear weapons. The only way to get that is not to use them.