A dirty bomb is like kissing your sister for an IslamoFascist. Seeing NYC go up in a great big fireball is the wet dream of these sickos - the fulfillment of their prophecies.
If 9/11 taught them anything, it was that when you strike a sleeping king, you better cut its head off in the first blow. Nukes would have likely taken us off the world economic and political stage for 10 years at least, and maybe would have done it permanently.
Look at the moral equivalency idiots (leftists) in our own country do on losing the WTC? Some have even argued that we deserved it because of all the 'enemies' we've been making abroad.
Nuking a couple of US cities would have eventually resulted in lefties and euroweenies saying, "Well, they lived by the sword (Hiroshima and Manhattan Project), so they died by the sword."
Watch the British movie "Dirty War" to see this in action. It's horrifying.