Simply because Japan cannot become the plaintiff for Senkaku since Japan already administrate them.
Stop acting as if PRC is a victim of some conspiracy. The 193 member nations of United Nations is not try to confine PRC and the US does not hold some ultimate control over IJC.
As I had posted before PRC simply does not have a leg to stand on in a court of law to justify their claim so they are trying to evade it as if it was the plague.
Pathetic attitude from a nation that claims they are the next global power.
A conspiracy is a 'hidden plot'; given that the US doesn't even try to hide their attempts to contain China's influence, it hardly qualifies as a 'conspiracy'. Indeed, the IJC is a plague which, as you have pointed out, is not under the 100% influence of the US - but plagues don't need a 100% fatality rate to be deemed hazardous.
You allege China has no claim over Diaoyu Islands despite concrete historical evidence to indicate Chinese ownership prior to Japan's robbing of them. Who would have thought a Japanese would be so blatant in selectively disregarding historical facts... oh wait... I take that back, no surprise there.