Wikileaks has always been as neutral as reasonably possible, given that it consists of human journalists and their human sources, not emotionless impartial machines. They have an excellent accuracy rate. The only thing that changed is that their honest reporting became embarrassing to wealthy and powerful people closely associated with the Democratic party. The DNC leadership loved Wikileaks' reporting on governmental malfeasance prior to late 2008. What changed in late 2008 was not Wikileaks' demonstrated neutrality towards reporting on governmental malfeasance, but the color of the lawn signs of the victorious president.
I suspect there's also some deep resentment from the traditional J-school cohort, who have spent over a decade being embarrassed by outsider Assange and his colleagues. Wikileaks has consistently make them look like obedient stenographers to the powerful, not principled journalists willing to speak truth to power. Wikileaks has publicly exposed many in the media as being too afraid to break real stories for fear of government legal pressure and a reduction in corporate advertising dollars. Raytheon, Lockheed-Martin, and Boeing don't advertise their military hardware on the cable news networks because they're trying to sell cruise missiles and fighter jets to the folks at home. They do it to hold something over the cable news networks' execs' heads when embarrassing stories might come to light.
Combine the collective resentment of the leadership of both major American political parties and their connected donors, the national security apparatus, the military-industrial complex, and every mainstream journalist who's getting scooped by an Australian ex-black-hat and his friends who (gasp!) never even attended Columbia, and we end up with a vicious and unfounded anti-wikileaks narrative festering within the politically-acceptable conversation today.