Why do mountain gorilla eat just 1.7% fruit, while lowland gorillas eat 25%? > Eastern lowland gorillas have more diverse diets, which vary seasonally. Leaves and pith are commonly eaten, but fruits can make up as much as 25% of their diets. < > The mountain gorilla is primarily a herbivore; the majority of its diet is composed of the leaves, shoots and stems (85.8%) of 142 plant species. It also feeds on bark (6.9%), roots (3.3%), flowers (2.3%), and fruit (1.7%) <

Mountain plants have to be hardy, especially if they grow above the frost line. They are more likely to be tough vegetation (leaves, shoots, and stems) as mentioned in the mountain gorilla diet.

The lowlands have much more plant diversity (and generally better food). Sweet fruits have better calories for the effort required to forage and digest them, so you would expect the lowland gorillas to prefer fruit.