Perhaps because it might not have been about money... if they used a CIA/NSA hacking tool to do it, it could easily have been a form of rebellion against the state. Also, Hackers are an egotistical and arrogant lot, and doing something big that gets recognition in their community is a very good motive for engineering something like this.
RE: The Daily Economist update for 15 May 2017 - Ransom me some Bitcoin