Because it fits the narrative. Tim Cook is being cast as the operations guy that may be a good manager but isn't a transformative leader that Steve Jobs was.

The title of her book is Haunted Empire: Apple After Steve Jobs. This is a book that could only be written 5-10 years from now, but who knows where Apple could be then. She's striking while the iron is hot; if Apple is irrelevant and no one is interested in the company in 10 years, she wouldn't sell nearly as many books.