Paraphrasing from Stephen jay Gould here: evolution is not directional, and big brains are expensive.

Some additional explanation: most of the time there is some sort of new physical barrier between populations of a species which keeps them from breeding with each other for a long while which causes a speciation event to occur. While it might be useful for either population to get smarter, genetic drift is totally random, so they stand roughly equal chances of getting smarter or dumber. To get smarter you have to invest more energy in child rearing, stay in the womb longer and develop a whole host of other adaptations to make it work. Meanwhile, the selection pressure in the environment has to consistently make it worth this additional investment. Big brains also use a lot of calories, and being smart doesn't necessarily mean more food or higher quality, higher nutrition food for a population of animals. If any particular genetic change makes an individual smarter it does not necessarily translate into a greater advantage for the population to propagate that variation.