How did control of Scientology fall into the hands of David Miscavage?

The founder of Scientology, science fiction author L. Ron Hubbard, had a wife and several children who presumably could have carried on as cult leaders after his death. Yet somehow the leadership of the cult fell out of the Hubbard family hands and ended up firmly in the grip of David Miscavage, who has parlayed that good fortune into a fortune rivaling that of the world’s wealthiest captains of industry. How did Miscavage end up the sole heir to Hubbard’s riches?