...because it's also related to "jejunum," the second portion of the intestines (duodenum, jejunum, ileum, colon):

from an online etymology dictionary:
jejunum (n.) late 14c., from Latin ieiunum, neuter of ieiunus (see jejune)...So-called because it typically is found empty during dissections, perhaps because it would tend to drain in a body laid on its back...