I believe that Kaby Lake brings support for LPDDR4, whereas Skylake doesn't have it. Apple only uses LPDDR on its laptops, so you can't get 32 GB as this would

I believe that Kaby Lake brings support for LPDDR4, whereas Skylake doesn't have it. Apple only uses LPDDR on its laptops, so you can't get 32 GB as this would mean DDR4 and higher power consumption.