WikiLeaks has shown great interest in anti-US material, and comparatively very little interest in anything that disparages Russia. Their bias has been analysed pretty thoroughly, and it calls their motives into question.
No, not necessarily. You're assuming that it's equally easy to get damaging materials from both countries. That's an incorrect assumption.
The explanation is simple: the US government does a horrible job with data security, and the Russian government is actually competent at it.
Anyone who's worked in or with the US government and dealt with USG computer systems and IT people knows just how incompetent they are. It's no wonder they're constantly being hacked. And also remember, here in the US, if you're a competent computer security professional, there's plenty of great-paying work for you in the private sector, since the tech companies are doing so well in this country. Why would you waste your time going to work for the government for lousy pay and having to deal with ridiculous bureaucracy? You can't even purchase a $5 piece of equipment you need for your job without filling out a bunch of forms and then waiting 6 months (I'm not exaggerating). So the only people who take these jobs are the ones who are utterly incompetent.
Over in Russia, things aren't the same; they don't have a booming tech sector like we do, so it's probably a pretty good move to go to work for the government there.
If hackers can easily steal emails from US government systems, but Russian government systems are locked up tight, then it stands to reason that WikiLeaks would post lots of US stuff instead. You can't get blood from a stone.