Maybe they wanted to avoid risk of not able to fit WS15 with current J-20 with WS10 engine design. Just look at J-10A vs B & C model differences. Many parts that are not in common and J-10A can't be modified into J-10C. Imagine if China already built many J-20 with WS-10, when J-20 with WS-15 came out with different design, there's less budget left to build more J-20 with WS-15.
Yes that would be the only reason not to maximise J-20 production now.