Matt Steinglass's guess:
It seems to me that the American and Israeli obsession with Iran's nuclear weapons programme proceeds from a misguided messianic-apocalyptic streak in both countries' political cultures. There's a temptation to imagine the world of foreign policy as a broad extension of a Robert Ludlum novel: a desperate time-constrained race to stop evil madmen from committing atrocities. This vision is morally clarifying and inspiring. But it has little to do with reality, and it distracts the public from the actual challenges of foreign policy, which are usually messy and often involve actual sacrifices in order to achieve publicly valuable goals.
Larison proposes an alternative theory.
The Dish 