Why do you say that? Did the U.S. and Britain use every weapon in their inventory when invading Iraq?
Besides, a dirty bomb would result in few if any more immediate casualties than any conventional bomb of the same size.
The main effect would be to make the area off-limits for a period of time.
The US and Britain aren't having their command and control structure steadily chopped to bits, and thus aren't under the same use-it-or-lose-it pressure.