Eh, whenever I see keepers training animals for strictly maintenance purposes, the huge crowd gathers. I seen this when keeper in Berlin trained wild horses to show foot etc. And when keepers washed elephants it was really crowd. Of course, keepers pretended not to notice visitors but puffed with pride
Hint to any zoo workers here: what really draws people is a sight of person interacting with animals. Any form. The only better thing is if visitors themselves could interact with animals. In any form, so simple as letting people feed antelope through the fence is extremely popular.
That are my observations from European zoos. I feel some zoos with circus shows sort of push themselves into the dead end - people expect more and more shows and ignore everything else.
I think it's more that people just like to watch the animals doing something different. You go to watch zoo animals and most of the time they'll be sleeping, eating, or wandering around. Have them do something different and people get excited. San Antonio Zoo got some tiger cubs a while back, very playful. When I would go, they would always attract a huge crowd since they were always running around playing and doing something silly.
Agreed on the circus loop. A lot of places make it too much of their selling point and it becomes hard to back out.