The better question is: What makes you think they ever did?
How the giant panda lost its taste for flesh
HOW did the giant panda lose its taste for flesh? The answer may lie in the gene that codes for the umami taste receptor.
It seems that pandas have an inactive version of the Tas1r1 gene that allows us to taste umami, say Jianzhi Zhang at the University of Michigan and his colleagues.
They discovered that Tas1r1 stopped working about 4.2 million years ago. Fossil records show that ancestors of the giant pandas swapped meat for bamboo between 7 and 2 million years ago. The team suspects the bears were caught out by environmental changes that wiped out a lot of their prey. When pandas changed diets the Tas1r1 gene became obsolete and, without it, they might not have wanted to eat meat even when it became plentiful.
But there’s a catch. Error bars in dating mean it is difficult to say which went first: the meat, or the panda’s taste for it.