According to The Daily Yomiuri, lack of food is not the only reason for bare grocery store shelves.  Even food that is abundant is not making it to the Japanese grocery stores because of a lack of packaging.

The problem is a lack of plastic, and plastic wrap, used for food packaging. The 11 March 2011, 9.0 quake severely damaged a petroleum complex in Kamisu, Ibaraki Prefecture, where the chemicals are made for plastics. (another example of  how we are dependent on oil)

Even plastic bottle cap producers have shut down.

Cardboard production has been stopped as well. The result is products like milk can not be packaged, according to Nippon Paper-Pak Co, a major beverage packaging firm.