There are a lot of reasons, but I think this article does a good job of listing the main criticisms: https://www.themarysue.com/the-autism-speaks-controversy-explained/
But in a nutshell:
- They frame autism as a disease that needs curing
- None of their board members are autistic
- They spread fear and misinformation about autism
- As a charity, embarrassingly little of their money goes to actually doing anything for autistic people