@Tigress, visitors leave your zoo just before they hit the mark of being in the zoo for 60 minutes (as displayed in their stats). So, if they didn't have time to see everything before being in the zoo that long, they leave with that as one of their thoughts.
In most of my zoos, I see this thought almost exclusively in larger groups of visitors where all they did was go to a bunch of food, drink and souvenir shops. I think what's happening is that each different individual of the group wants to go to a different shop so they end up spending almost their entire zoo visit just in the lines, then run out of time.
The only things we can do to minimize this is have enough shops and highly trained vendors such that lines aren't long and don't make the zoos so big that a lot of walking sucks up the visitor time as well. That last one is probably why it's so much easier to keep everyone happy in smaller zoos.
My personal opinion is that Frontier needs to fix how these large groups just go to shops. If the group is in line at a drink shop, everyone in the group needs to buy a drink there so they don't need to go to another one soon. It seems the easiest way to fix the game to give the larger groups enough time to also see animals. I feel like they made a change in one of the recent patches that is opposite of this...because I see these thoughts way more than I used to.