Found on a rubbish heap? After being received as a gift to the gods from a fabulously wealthy king?
Maybe the priests thought it was similar to getting a canned fruitcake from someone for Christmas that has a date of 1945 on the bottom.
The wood frame probably either rotted or was burned. Delphi was closed by one of the Roman emperors, and had been business for centuries prior to that, so the wonderful gifts gradually became junk while setting around the temple.
http://www.pbs.org/empires/thegreeks/keyevents/1400_c.html
http://news.nationalgeographic.com/news/2001/08/0814_delphioracle.html