There are things that simply must never see the light. Dark and evil things that grow in EVERY man's private corners but are suppressed by good men. Given just a little awareness and intention, evil people that would resurrect this madness, and would take those seeds and nurture them for years. At the right moment they'd then pounce and resurrect all of this. Better to keep it locked away.
Or it could be simply that there's so much solid evidence that the US and others have done things that resulted in millions being killed. Even with the perpetrators revealed (some already dead), those victims' families would sue the US and others so badly there would no longer be a US.
I think that's the gist of why Q and others might say no. Just my gut feel.