The reason they sounded alike is because both cartoon characters were Hanna Barbera characters, voiced by the same voice-actor - Don Messick. Don created the now so familiar voice pattern, and used it for both Scooby and Astro. Don by the way, did many-many voices from the Hanna Barbera cartoon menagerie.
Later in life, Messick said he could no longer do his most famous voice because he had quit smoking, the rasp that made Scooby - Scooby was absent.
Personally, I always favored Astro over Scooby-Doo. My 10 year old daughter would beg to differ.