Secondly, because clean nukes need to be so huge that they are simply too dangerous for their utility. Think at minimum "more powerful than all conventional explosives ever produced" level huge.

While doing things like channeling to the Qattara depression sounds promising, the actual practical problems of operating with true city-busters are just too great. There are plenty of industrial accidents caused by explosives every year, you don't want to live in a world where a really bad accident can wipe out a city.

There's a reason the world looks in askance when nations start enriching uranium, even if it can be used for good.