Andromeda left because she fell in love with a muggle-born -- but I'd love to explore that she only learnt of her still-there prejudices towards them after they got together. I'd like to think that Andromeda was prideful, and stubborn, but because of this it led to her believing she no longer held any sort of negative thoughts towards muggle-born and them being in the community.
When she first got with Ted -whether she liked it or not- he was her exception. Whether she liked it or not, she thought this made her exempt from any thoughts she had towards muggle-born that were hateful/stereotypical. This led to a great fight between her and Ted once he picked up on it, and only then.
Did she begin to learn and become the person she is. She raised a head-strong, stubborn, amazing woman who fought for what was right even if it costed her life, and a future with her son and husband. She raised a loyal, kind-hearted woman with a great sense of justice, even if her husband was a hufflepuff, she still helped raise a hufflepuff.
But she was only capable of raising this woman after she accepted that she wasn't perfect and she did need to learn and gain the facts -not just the opinions of those around her.