From your post I take it you don't like the idea of animals in small enclosures rather than them being ignored.
For what it's worth my view is this:
Lots of the animals you mention are usually attractive to children, who can relater to their small size and odd appearances, but adults tend to want to rush on to see the so called rarer animals.
Personally I feel a lot of the time it is down to education, for example in the wild the axolotl is one of the rarest of the worlds amphibs as the only place it lives are la couple of lakes around Mexico City which thanks to man, have been so degraded and drained, coupled with introduced fish species for fishing that prey on the axolotl, they are now very near extinction in the wild state.
In fact there are probably more of them in captivity than ever lived in the wild, but I bet the zoo doesn't tell the people that, if they did more folk would actually look at and appreciate these exhibits. Most zoo's just fit things in were they can and sometimes the theme doesn't go together so is overlooked.