Time? But besides that, I can't really think of anything. Some guy on NPR was talking about how it might be some professional developers who did this in their free time, so that would kind of make sense.
But the problem with later is that the more people know about it and the longer it takes to make such malware, the higher are the chances of a pre release leak, or some double agent doing his work. So one or two very intelligent people doing it in very small time make more practical sense for a covert mission.