Maybe it is just exceedingly hard to provide meaningful education for the average member of the public visiting a zoo ?
It can be, yes. And I say that from experience. Maybe 20-25% of the people at an exhibit read available signage. I would dare say less than 5% actually read the whole thing. Many times I've listened to an "expert" guest rattle off multiple incorrect facts to their companions while literally standing right next to an educational sign.
I think that much much more could be done in terms of exhibiting and showcasing smaller and taxa of conservation concern such as small mammals, small birds, reptiles, amphibians, fish and inverts from many different biogeographic regions (I would rather zoos shifted to this focus rather than the megafauna craze
Megafauna draws people and pays the bills for many zoos. Most often Joe and Jane Public go to the zoo for the tigers, elephants, giraffes, bears, seals, and other large, "dangerous", well-loved species. Now there are plenty of exceptions to this; fish-oriented aquariums, bird parks, reptariums, and small local facilities. But as a whole megafauna carries the wow factor than finches and beetles do not. Given how many of the commonly kept megafauna species are endangered, they are just as deserving. Several megafauna owe their existence to zoos, and this should not be overlooked.
I am not saying zoos cannot/should not focus on small species; rather that by coming to see the megafauna, the public can be introduced to smaller and lesser known species as well. Animal ambassadors are often key in this regard, especially for reptiles since many people fear them.
native species and their conservation is always a wise management / curatorial choice and I believe that most zoos don't do nearly enough in that regard
I think many do, though it may not be obvious much if the time. Species such as Grizzly Bears, river otters, Bald Eagles, and alligators are commonplace here in North America, and the public loves getting to see them up close. They often know the species lives near them but many have never seen a wild one. Exhibiting these species helps garner awareness and interest. Otters are everywhere, but kids adore their antics and love interacting with them.
There are quite a few native endangered species being worked with and promoted in North American zoos, including California Condor, Sonoran Pronghorn, Red Wolf, and Whooping Crane. Numerous species are raised by local zoos for release, including frogs, salamanders, and even fish. SDZ Global always announces the first Condor egg and first hatch of the year, supplemented by the live cam watching Condor chicks grow up. I know of multiple smaller programs going on that participating zoos sporadically announce news on. Awareness could be better for some of these smaller programs, but they are at least occurring and being promoted occasionally.