We don't know WHY NVIDIA wasn't able to secure 7nm, but it is the better node and they had to settle for Samsung. It's possible at the time it didn't look like TSMC had enough capacity, remember NVIDIA outsells AMD 3:1 (or more), so enough capacity for AMD != enough for NVIDIA. Also, it's not just AMD capacity at TSMC, they are only 30% of the 7nm capacity and that's only because Apple moved to 5nm. Also, they didn't know the Huaweui thing was going to happen either. I think NMvidia made the decision they made based on price (TSMC is more expensive because they have a better node and more people fighting for it) and capacity at the time. That said, I absolutely expect to see more availability of NVIDIA cards mid-late october, not beacuse they are holding them, just because this was a rushed release and the channels will finally be seeing some products. The early/rushed release was just to make sure they were first, knowing they didn't have near the amount of product for a real release.
except that they did secure capacity.. TSMC is producing the A100 chips, but likely the decision to go samsung for the consumer cards was cost related. samsung underbid and likely over promised their capacity for 8nm.
ultimately though i think they ended up releasing far too early, the release date was likely set internally way before they knew what the availability of chips was going to be and didn't want to change it.