The leak can get to multiple journalists and NGOs. If the story is a human rights abuse in the US and the New York Times refuses to publish it, the story will still get told if it's revealed to Der Spiegel, the Guardian, the Red Cross, and the UN Human Rights Committee.
Of those willing to look at leaks, journalists are more likely to have the connections to properly redact sensitive documents.
Journalists are typically the ones that sift through large accounts such as the 250,000 cables in CableGate to disseminate the interesting information. Historians and political scientists tend to be the others, but they lack the ability to publish as widely as journalists.
Journalists in most countries already have legal protections.