>about primates that have high carb-sugar diets but doesn't explain why their brains remain so small compared to humans. Maybe they will eventually evolve?

if they have abundant sources of carb-sugar, there would be no selection pressure to evolve in a particular direction.

Don't most evolutionary traits/changes come at times of restrictions? I don't think many traits are developed during times of abundance, it's when there is a shortage and they have to seek out something else; like protein. Or, sometimes it's just natural competition for the share of the abundance. If there where a big brain out competing you for your carbs, you might have to reconsider your dietary options and make substitutes/changes that would have evolutionary implications.