What on earth made Jerry Van Dyke think that this was worth pursuing as opposed to playing Gilligan or being Barney's replacement on The Andy Griffith Show? Was it a chance to star in his own series? Did the script really sell him? I'm just curious

Maybe he felt the choice was being on a show that actors were stuck in one place (Gilligans Island) and how long could that last. Gimmick shows like Bewitched, Mr Ed, I Dream of Jeannie were doing well. As least you could change locations. He might have thought if a talking horse like Mr Ed can make a go, a car that talks might too.

Also, in hindsight Gilligan will never be forgotten, but there's no way to know that at the time.