The smartest animals, such as elephants, dolphins, and chimps, have long generation times, high expense, and some protection from experimentation. For animals with shorter generation times, one has more distance to travel, and while cool experiments are possible, their timescales still wind up being long. And we have much larger potential datasets for human genomes tagged with cognitive data (collected for medical and educational reasons, so not posing much marginal cost) than for other animals.