Because Adam "hearkened unto the voice of his wife."
Thorns also and thistles shall it bring forth. Genesis 3:17
And unto Adam he said, Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree,
of which I commanded thee, saying, Thou shalt not eat of it:
cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life.
Thorns also and thistles shall it bring forth. Genesis 3:17