Surely the largest part of the "blame" has to lie with the US authorities for putting such sensitive cables somewhere where so many people (Bradley Manning being only one of them) could access them?
It was a calculated decision to greatly increase information sharing between government agencies (including the military) to allow for better cooperation.
My suggestion to anyone with secrets that valuable is to assume they get leaked and have a plan for that.
I'm curious if Wikileaks saw this coming, but I suspect they were naive enough to believe they could maintain control.