Well, imo same rush that resulted in the PLAAF 6th gens flying years before the american ones. Whatever the strategic reasoning for PLAAF was, it applies exactly the same for PLANAF, quality over quantity being one of those factors probably. It's not like the J-35 will be displaced too early, we are looking at probably almost a decade of sustained production until J-50H(?) should be ready for sustained production itself. But even for this pretty conservative timeline they need to step on the gas now.