I'm not sure how that is relevant and I believe the declassified material you are referring to was an NSA cable sent on the day of the event, when very little was known. Wikipedia has details on the various sources for anyone interested.
The Nanjing Massacre was a horrific incident and I would urge anyone visiting Nanjing to visit the memorial.
That said, one incident was a wartime civilian massacre from an invading army where that army has been defeated and is largely a different country now. The other was the killing of protesting civilians by their own government, which is still in power and broadly unchanged.
Given this, it's clear why one is more topical than the other