They're just pretty shitty as an organization. Very focused on the neurotypical parents of autistic children, very cure-oriented, no autistic people involved in their decisionmaking, they spread harmful memes about nasty "treatments" and have a history of spending way too much time looking into vaccines on a cause. Most autistic people don't like 'em, is the short version. Googling "autism speaks criticism" or similar will get you lots of specificity.