I second this. To your question, I believe it's because the notion is multifaceted, and there are many reasonable answers. The most common being that we need nuclear weapons for deterrence (defensive) reasons, same as the reason for having a war machine or military alliances at all. A reason less touted is that we need it for offensive reasons, but then again, the best defense is offence so the two are difficult to clearly distinguish. Another reasonable argument against a ban regards enforcement: how to you enforce something from a position of weakness (having vowed not to use nuclear against a ban-breaker)? I could go on but the point is not that these reasonable objections are insurmountable obstacles, but rather that there are real difficulties that need to be addressed in order to avoid the nuclear havoc that otherwise seems a probable end result some time this century.