Manning used her security clearance to steal the cables. They were not emails and the controversial stuff was not authored by Clinton. They were candid (because supposed to be secret) assessments of foreign situations, and candor can be embarrassing or even harmful if made public. This was when I knew Assange was not so much a crusader for truth as an enemy of the U.S. And don’t get me wrong, I’m not a flag-waving uber-patriot, but I also don’t want my country harmed for no good reason.
TL;DR what steronz said.
ETA, most of the material dated from before Clinton was SS.