Idk if "descendants of omnivores" counts because then you could exclude a number of critters like pigs for being "descendants of herbivores" and then 'why do pigs have more stereoscopic vision than a t-rex '
The obvious caveat is that pandas at the minimum don't have selective pressure for side eyes or they have something pressuring stereoscopic vision even more similar to how aquatic animals have less selective pressure for forward facing eyes.
I would imagine the way pandas eat bamboo stalks is more visual than most herbivores and that alone could help them retain steroscopic vision.