Why do mountain gorilla eat just 1.7% fruit, while lowland gorillas eat 25%?
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.