Sinclair International, seller of reloading supplies, have explained the lack of primer availability on their blog (emphasis mine):

The first driving force has been the huge demand for loaded ammunition through 2008 and continuing into 2009. The large primer manufacturers like ATK (which would include Federal and CCI), Winchester, and Remington are directing the majority of their primers into loaded ammunition.

…

the result has been consumers stockpiling and hoarding primers. Individuals are buying and keeping more primers in their own personal inventories and this has prevented some reloaders from having any primers at all. We normally see people buying 1,000 or maybe 5,000 primers at a time, now we are seeing customers buying 25,000 at a time.

…

Federal told us a couple of weeks ago to not be surprised if we don’t see many primers from them until the end of 2009.

A very dire situation indeed. I expect few of us will ever forget the ammunition drought of 2009!

I highly recommend reading the whole blog entry.

Thanks to Commodore for emailing me the link.