Why are many endangered species at risk due to genetic drift?

I would think of genetic drift to go hand-in hand with environmental adaptation. So, I wouldn't think it would cause endangered species. However, it also creates subspecies. For example, there is the "Northern White Rhino" and the "Southern White Rhino". The two are very similar, but considered different species due to population isolation and millennia of genetic drift. Unfortunately, there are now no known remaining Northern White Rhinos in the wild, although the southern cousins are quite numerous. The Quagga is a Zebra subspecies that outwardly has a different striping pattern from other zebras. And, apparently it is also extinct. There is a current breeding program directed at breeding quagga features into a zebra herd. Reintroducing a species such as wolves or bears might also be considered as reintroducing a similar, but not quite the same species. Introducing a non-native species, of course, could cause interbreeding with native species, or out competing of the native species. Presumably this wouldn't be a bad problem unless lethal genes would get introduced. Sometimes this can be a bit inadvertent. My parents used to have some wild white geese on their property (came over from the neighbors). It was kind of neat that their pasture became a favorite stopover for the Canadian Honkers, presumably attracted by the domestic geese. That is, until a few years later we started noticing geese with white spots stopping by.