It's an evolutionary trait because bitter often meant poisonous. Women are also more prone to the sensations of bitter tasting foods than men due to their role in picking out backup meals if there was no successful hunt.
Apparently this protective trait stopped giving a fuck around a human's mid twenties for some reason. I haven't read into why something designed to protect you from eating something that could make you sick suddenly decides "I'm done, eat what you want, faggot."
