http://cryptome.org/0002/ja-conspiracies.pdf
To me the answer is that America's current "Left wing" is more in-line with the American Neoliberal world dominance agenda. Meanwhile Americas conservatives where they promote American dominance do so in public. Republican candidates debated candidly about how much they'd like the "bad guys" to get tortured. Meanwhile the DNCs favoured candidate got big donations from the defence industry and Saudi Arabia whilst talking about how important human rights are.
Wikileaks project is about attacking powerful in-groups deception of voters. All leaks help this cause by increasing the info-sec cost of conspiracy. But if the in-groups private messaging is in line with their publicly stated positions then what's there to leak?