I'm assuming that as populations become larger (more successful?) that the selection process becomes more diffuse. It's clear that sub-populations have evolved or acquired useful traits (particularly dietary adaptations) in relatively short timescales.

From TFA it appears that the original human population in Africa was pushed out of the interior by climate changes, it's not inconceivable that those conditions also provided evolutionary pressure

> why did whatever selective process that resulted in our capacity for thought stop improving on it 100,000 years ago?

Some people have posited that it's because some fundamental biological limits were run into. (Our biology has already made many compromises to support the large brain, such as extended childhood, compromises in women's skeletal structure, the large energy consumption of the brain, etc.)