I think that they decided based on where they were in their lives and what sort of roles they wanted to further their careers. And let's face it: the actors that ended up doing those roles were a PERFECT cast. There is no way of telling if casting these other actors would have resulted in hits as big as the final products ended up being. But...there is a reason Harrison Ford IS Indy, Brian Cranston IS Walter White, etc. They were THE ONES for those roles.