Because of mass, the number of neurons, how many calories we can toss at them, and how many calories a fetus/newborn can be guaranteed.
Even if you have trainable brain, with neurons that can communicate with more percentage of the brain than a primates. You're still on a diet of hundreds of calories instead of thousands.
Looking at smaller animals cognitive abilities is newish. But does look like it will be going interesting places.
How come that primates like humans, are much smarter than the rest of the world?