Last time I was in China was around this time a year ago, in Beijing, and yes, though I can open up the SDF main page I couldn't go any further. Take it as it is.
As to the "why" part, though SDF is relatively pro-China when comes to a foreign forum (I don't know who exactly foots the bill for SDF so I can't use the words "purely foreign forum" ), we do throw around sensitive topics (to Chinese sensors, that is) here so that might be that.