A piece in Quartz explores this question:
In many cases, you could stand at the edge of one village and see the outskirts of the next community. Yet the residents of each village spoke completely different languages. According to recent work by my colleagues at the Max Planck Institute for the Science of Human History, this island, just 100 km long and 20 km wide, is home to speakers of perhaps 40 different indigenous languages. Why so many?
We could ask this same question of the entire globe. People don’t speak one universal language, or even a handful. Instead, today our species collectively speaks over 7,000 distinct languages.
I learned a number of interesting things about the dispersion of human languages from this article.
peakmemory 
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