TL;DR: rapidly expanding populations have faster adaptive evolution, and that’s what is seen in humans.

> Human populations have increased vastly in numbers during the past 50,000 years or more (1). In theory, more people means more new adaptive mutations (2). Hence, human population growth should have increased in the rate of adaptive substitutions: an acceleration of new positively selected alleles. … In such a transient, large population, size increases the rate and effectiveness of adaptive responses. For example, natural insect populations often produce effective monogenic resistance to pesticides, whereas small laboratory populations under similar selection develop less effective polygenic adaptations (5). Chemostat experiments on Escherichia coli show a continued response to selection (6), with continuous and repeatable responses in large populations but variable and episodic responses in small populations (7). These results are explained by a model in which smaller population size limits the rate of adaptive evolution (8). A population that suddenly increases in size has the potential for rapid adaptive change. The best analogy to recent human evolution may be the rapid evolution of domesticates such as maize (9, 10).

—Recent acceleration of human adaptive evolution

The author of that paper has a blog post giving more background and explanation: Our new paper on why human evolution accelerated. His summary there:

> Our evolution has recently accelerated by around 100-fold. And that's exactly what we would expect from the enormous growth of our population.