I think it depends sometimes on the quality of water in the shops tanks, and how quarantine went as well for imports.
Different fish are susceptible to different things, imported guppies can carry disease that they are immune to but if you put them in a tank with locally bred guppies, the locally bred ones may die. Dwarf gourami seem to have problems. Clown loaches get white spot easily.
Imported fighters tend to be a reasonable age when we get them here.
Some fish simply live longer than others.
The list goes on..