"Whistle blower" also fairly strongly implies that what the government was doing is negative and should be called out, which the media tends to be hesitant to cal out.
I don't think leaker has mild negative connotations, and if it once did, it doesn't anymore because of the popularity of organizations like WikiLeaks. Leaking is in the name, after all.
You really don't think WikiLeaks has a negative connotation in the media and the public in and of itself? The media spent a very long time running WikiLeaks through the mud.
And yet WikiLeaks remains a fairly well-funded organization with a huge following that continues to get leaks - and a major source of stories for that very same news media you say dragged it through the mud.
"Whistle blower" has more positive connotations.