I genuinely think the reason for it is they breed these particular fish in huge quantities, and the genetics get all mixed up (or not mixed up). And this poor genetics cause more issues the older the fish. They only live 1 year in the wild, 2 or 3 years in an aquarium, so degenerate diseases start to show sooner or later. I have some zebra danios that are getting on for 3 years now and there is a noticable bend to their spines, but they still seem ok, albeit they have slowed down a little. I put some younger danios in last week, and it seemed to reinvigorate them though which made me laugh a little.
Zebra danios are also prone to TB. I just read that this is transmittable to humans which is a bit worrying.