Why do non-governmental organizations tend to publish non peer-reviewed "grey" literature?
I can think of a couple of reasons:


Peer review is slow. NGOs want their paper to influence policy now, not in one or two years from now.
Peer review journals have a different audience than grey literature. NGOs typically want to communicate with people like policy makers or journalists, and these are less likely to read peer reviewed journals.
Related to point 2: in order to effectively communicate with their intended audience they need a different style of writing and presenting their arguments than what is common and acceptable in peer reviewed journals.