If the 100 TB figure is correct, this has been going on for some time - it takes time to steal that much data in a way that does not raise a bunch of red flags. If the red flags weren't there to be raised or they were and were ignored, well... at least their executives got their bonuses.
Also, in the interest of fairness, while this malware attack seemed to be directed to Windows machines, a dedicated enough intruder would have developed attack strategies for any platform.