Its been mentioned elsewhere - its down to cost. TSMC wont discount and its nearly $10,000 per wafer for 7nm (and $17,000 for 5nm , Apple has 100% of TSMC capacity for 5nm). For Samsung that figure drops to around $6000 per 8nm wafer; therefore Nvidia came earn far more money even though they get less gpu cores per wafer. TSMC are sold out , which is why i genuinely dont expect Nvidia to move on to TSMC 7nm anytime soon, as its believed that when Apple close down 7nm - AMD have a supply contract already to use that capacity.