It was likely a genetic thing.
“Indeed a genetic basis for longevity has been demonstrated in animals, e.g. fruit flies, such that genes for longevity can be lost from a population.”
“Recent advanced computer simulations vindicate this proposal, showing that an exponential decay of lifespans fits well with accumulating mutations after the catastrophic population bottleneck at the Flood.”
https://creation.com/living-as-long-as-methuselah
Good point! I didn’t consider the Flood.