The original budget was 5 billion. I am curious what all changed in the design to make it triple (they still had many billions left to complete) the costs over

The original budget was 5 billion. I am curious what all changed in the design to make it triple (they still had many billions left to complete) the costs over the budget? was it crappy budgeting in the first place? bad project management? huge changes in requirements after Fukishama?