For the same reason they don't worry about American Musket fire. We have much bigger, nastier and more lethal stuff on the shelf. A dirty bomb is just radioactive material packed around a regular bomb.
A dirty bomb wouldn't likely kill that many people in the initial attack and most of those would be from the explosion it's self.
Over time there would be a rise in cancer rates but as either an offensive or defensive weapon a dirty bomb just isn't lethal enough to to interest us.
I meant as an anonymous, reverse terror weapon. I agree its not good for formal war.