Because the form of autism most people are familiar with is the high support needs versions, where someone will never mentally mature past being a child, and will need lifelong care.
ADHD doesn't have that counterpart, so it and lower support needs autism were simply seen as personality failures and eccentricities that could be corrected, or left in place without harm.
"If only you would concentrate harder and were more careful"-type business. A lot of people still have that association with autism and ADHD.
I don't know about that. It's more likely to have been for categorisation reasons. They're the same sort of disorder where severe Asperger's has strong overlaps with ASD, so they might have been bundled together to avoid confusion when it comes to diagnosis.