Why is the diet of a giant panda made up almost entirely of bamboo?

The giant panda's diet consists of 90–98% bamboo, including its leaves, shoots, and stems. Despite adaptations in their teeth and jaws for consuming bamboo, pandas have the digestive system of a carnivore and cannot digest cellulose, a primary component of bamboo. To compensate, they rapidly process large quantities of bamboo, spending up to 16 hours a day feeding and excreting waste frequently. This reliance on bamboo is ancient, dating back at least three million years. While bamboo is their primary food, pandas retain a taste for meat. Their habitat is exclusively bamboo forests, as they cannot survive outside of them.1