Because that’s what the units that have the most experience flying the J-20 have.
Yeah sure let’s use air shows rather than production schedules as our mark of technological confidence.
We have not seen Bai Long for a long time!J-20 pilots, crème de la crème of the Chinese Air Force.
Why did they choose to fly j-20 equipped with russian engines?
Possibly. Other reasons might be to reserve the newer WS-10X J-20s for training, or to send a show of solidarity with the Russians.¯\_(ツ)_/¯
Could be any reason, really. Maybe since there were about 15+ J-20s in the parade, there wouldn't be enough WS-10 jets to do it. Maybe the pilots flying the Russian-engine jets are more experienced than the WS-10 jet pilots.