Maybe, the PL-15 was not designed with the J-20 in mind and the PL-21 was. So, J-20 just have to wait for the PL-21.

I wonder if it is possible to fit 2 PL-10s on ONE side-rack of the J-20-- with the tail-fins of the PL-10 folded. Together with PL-21s, that could enable the J-20 to carry a total of 10 AA missiles.