People don't always doubt the credibility of Wikileaks. They dislike Wikileaks because while they be posting accurate stuff, they've also been picking and choosing what goes out, and have admitted communications with the Trump camp to damage Clinton while sitting on information on Trump.
Wikileaks loses credibility in the sense that they're not doing shit out of some sense of divine justice against corrupt political systems. They lose credibility because they're using their documents and sources to smear select parties while Julian Assange jumped into the pocket of the Kremlin when they threatened him.