I would also imagine that it's very much a cultural issue as well. You can't get these documents without people who believe strongly that they are making the world a better place by collecting them and releasing them.
Now the US has a lot of idealism going on, so much so that many people grow up idolizing institutions like the CIA and the NSA. They join these institutions as believers, but day after day they are exposed to more details on the inside that bursts that idealism and one day cognitive dissonance gets the better of them and they choose to collect information to become a whistleblower.
Compare this with other countries where there isn't as much blind reverence cultivated for such institutions. The same talented kids that would grow up under circumstances where they would have a much more realistic view of the good and bad these institutions are capable of. This in and of itself is a major limiting factor in how high you would rise in such an institution, assuming you chose to join in the first place. You can't be a whistleblower if you don't have access to anything to whistleblow about.
Whistleblowing happens when values clash with reality. If values are never really part of the equation, and you have organizations where "the ends justify the means" is not only accepted, but rewarded, then you don't get whistleblowers.
With things like Guantanamo, extraordinary rendition, drone assassinations, and all these realizations from the Snowden docs, I fully expect that idolization bubble to be pretty much popped by now insofar as institutions like the NSA and CIA are concerned. For that reason more than any other I expect whistleblowing to become less frequent for these institutions. If and when whistleblowing drops for these institutions drops, it's will be explained by better security measures, but I would expect it having more to do with those who have the personality to be whistleblowers never joining or rising high enough to have that opportunity.