1, J-20 production apparently hit the roof, and it interferes with Chengdu other operations and designs. Factory is overloaded.
2, Shenyang needs VLO production experience. Flankers aren't birds built to a millimeter precision.
3, Shenyang main aircraft product (J-16) is 10 yrs old, they need new modern products.
J-16 orders are likely close to completion, it isn't a new, fresh plane anymore.
4, J-20(a) has different mission profile when compared to J-35A. It is smaller, it almost certainly has a multirole EOTS(unlike J-20). It's very likely somewhat lower observable.
5, J-20(a) comes at a different price point, as well as higher purchase and sustainance costs.