Cost of metals rising is one factor. China and India have been buying a lot of brass, and copper lately so this has caused supplies to fluctuate and prices to rise. Google "copper thieves" and you will find many articles about crooks stealing copper wire from constuction sites due to the high value it has on the scrap market right now.
The U.S dollar is dropping in value, and prices on all goods are rising to match.
The laws of supply and demand. Uncle Sam is buying a lot of ammo right now for the WOT.
Last but not least one big reason is people buying ammo. Retailers are gouging us on prices, and as long as people are willing to pay $300 per thousand on 5.56 they will keep charging that much or more. They will charge what the market will bear.