I'm wondering if it's because autism can mean anything from what used to be called Aspergers syndrome and profound autism with profound learning disabilities and people with adhd are more likely to either not have learning disabilities or have learning disabilities with a separate diagnosis.
But Nigel Farage has been in the news again saying he is going to get rid of PIP for people with mild anxiety and adhd as their primary diagnosis. I don't think anyone gets PIP for mild anxiety, I certainly don't anyway. But adhd can be extremely debilitating and require PIP.