Asherah was much older than Yahweh and had been the primary diety in the region for thousands of years before he came along, going by Astarte, Ishtar, Innana, and other names depending on the location. She only becomes the consort of god in an effort to convert the people away from her worship to his. The serpent is one of her divine animals, so reimagining it as evil helps to turn the tide against her.