Visitors fluctuating in Zoo 2: Animal Park is a natural mechanic. Visitors don't like a neglected zoo. If you have animals in need of care, plants that need to be watered, or trash all over the place, less visitors will come.
If you click all of your animal care, watering and trash pins, you'll see the visitor bar fill again, and the number of visitors will start increasing again. Keeping your zoo ship-shape is key to your success!
If you want to increase the total possible amount of visitors you can have, be sure to get more decorations and animals!