What might be apparent, might not be true.
Secondly, when any air force goes for a new platform, they have an idea of what exactly they want, it doesn't matter how much R&D or testing that product needs. They want the best. The Blk60 is a prime example, its radar, ew suite, ram coating - haveglass, higher thrust engine, all this cutting edge tech nearly 15+yrs ago.
People did not understand the for R&D + propriety tech US$3bn that had to be paid, but the
Rafale's R&D for F4 from F3r cost US$2.3bn.
So the US$3bn for the Blk60 i.e. a foreign partner, is very much justifiable.
Anyways, Three things point towards why J15 and not J16:
1) The fiscal terms
2) The degree of customization
3) Maybe joint carrier ops
I am always open to correction, but my understanding is that the newer versions of J15 have more composites, than J16.