I have no idea, however, I would guess that in spite of all our hard work to replicate their natural environment, we haven't quite gotten it exactly right yet.
True it could have several factors the type off prey item the amount of prey offered, the nutritional value of the prey offered, Amount of rain, or lack their of, elevation above sea level, hell it could be something in the water.
When you've got 10,000 people trying to do the same thing, why would you want to be number 10,001? ~ Mark Cuban