Because Gorillas Are Big.
“Large mammalian herbivores, according to accepted ecological theory, may be expected to feed on abundant, low quality food as a result of the relationship between their body size, metabolic requirements, and gut capacity. … Abundant low quality food means nonreproductive plant parts, i.e., stems and leaves, because fruit and flowers are too ephemeral to provide a regular source of food in bulk for large animals. So, large herbivores ought to be folivorous in a broad sense, and usually are.”
In other words, big plant-eaters mainly eat leaves.