A major reason for the decline of suicide and roadside bombs in Iraq over the past year has been a shortage of bomb making materials. For example, in the past month, the Iraqi army found and destroyed three tons of explosives, 600 blast caps and a large quantity of primer cord (for setting off several explosives simultaneously), 26 grenades, 25 shells of various calibers and one completed bomb. Until two years ago, U.S. forces were finding and destroying this stuff. But now Iraqi police and troops are getting most of the growing number of tips on where remaining bomb making materials are. Some of this stuff is owned by terrorist groups, but a lot of it was grabbed by Iraqis hoping to eventually sell it to someone. Some of this material has been hidden away for six years, either still waiting for a buyer, or its location lost when its owner was killed.
Since nearly all Iraqis fear the bombs (which are used by Sunni terrorists against Shia and Shia terrorists against Sunnis), they all have an incentive to tip off the cops. Thus the fear, generated by several years of indiscriminate bombing, the proliferation of cell phones, and Iraqi police and soldiers now stationed throughout the country, has made it more difficult to hang onto bomb making equipment. Detonators (blasting caps and primer cord) are a lucrative for smugglers to sneak into the country, but more and more bombs are being fabricated locally from common chemicals (including fertilizer). The locally made explosives are less powerful, and easier to find. But, in general, it's gotten harder to gather and store bomb making materials. This, in effect, has crippled the enemy supply line, in that the terrorists have much less access to the weapons they prefer to use.