The J-20 has canards so there is likely less improvement in maneuverability in going with TVC in it to begin with.I wonder if the lack of TVC is just for early trials or whehter PLAAF have determined that the additional maneuverability conferred by TVC simply isn't worth the extra procurement and maintenance costs and the reduction in thrust.
TVC also decreases TBO for engines significantly. It is another point of failure in the aircraft.