The reason isn't just about peanut butter and jelly, it's the fact that it comes from food that is grown, not from meat products. Any plant-based meal would do, but the PB&J Campaign suggests having a peanut butter and jelly.
April 2 marks PB&J Day
April 2, 2010 According to the campaign, eating a few PB&J sandwiches a week can help shrink your carbon footprint.