......Too tight a schedule.
The root cause for these delays is more likely influenced by overly ambitious time-tables, a new manufacturing process and the use of technology and materials once restricted to smaller scale projects.
In short neither Airbus or Boeing have allowed a sensible amount of time for the development of their aircraft - both step changes in current technologies/products.
Boeing didn't move its HQ from seattle to Chicago for nothing... I suspect it was very much along the lines of getting the engineers out of the board room and this is the result.