What are the reasons that people blatantly ignore clear scientific data that autism and vaccines are unrelated?

Hope and blame: Autism is a terrible condition. When advanced there is little ordinary love and affection between the parent and the child. Parents of such a child desperately would love to understand a cause which could be acted upon with their child and to help others. The totally discredited data relating autism to vaccines still offers hope to them and they continue to believe despite the science.