I'm sure if he had dirt on Trump he'd release it also.
At one time, I would have agreed with you. However, it's becoming clear that Assange is not simply freeing information; he's playing politics -- possibly in hopes of a Trump pardon. That's not a far-fetched goal. Assange's standing among conservatives has improved greatly since he started dumping on Clinton. Take the example of Fox News anchor (and Trump lapdog) Sean Hannity. In 2010, he was calling for Assange's head and castigating Obama for not taking out wikileaks. Now, Hannity wants Assange to go free. (Source) So, if Assange had dirt on Trump, I highly doubt that Assange would release it. He wouldn't want to alienate his most powerful audience.