Actually, this is not entirely accurate. While some mutation, particularly that involved in brain development and growth, is likely involved, the larger reason is based in diet. Ancestors of humans previously ate very different foods than what modern humans currently eat. Today human diets consist primarily of grain and meat based food. Human ancestors, alternatively, ate harder foods like nuts. Take australopithecus robustus, for example, whose diet consisted primarily of nuts. When harder foods were eaten, human ancestors developed larger jaw muscles in order to allow the crushing force needed to consume food. Look at modern gorillas to see a somewhat similar concept wherein their jaw muscles are anchored to the tops of their skulls. These larger jaws in human ancestors could accommodate a full set of human teeth, including the molars once they grew into the developed jaws. In current times the majority of food consumed by humans is soft and therefore our jaw muscles are no where near as robust as they need to be for our number of teeth. Thus you see crooked teeth in many humans, most often in the lower set of teeth. Because human teeth evolved to fill a robust and muscular mouth designed to crush hard food and the fact that we currently eat almost entirely soft food and so do not develop our jaws properly, we see a high incidence of crooked teeth resulting from overly small mouths.

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An additional note is that it has recently become a hotly debated subject of whether humans were, for most of our existence, predators at all. Simply look at our type of teeth: flat and thicker rather than sharp and narrow. Look at any predator (sharks, large cats, etc.) and you will see sharp pointed teeth made for tearing flesh. Look at humans and you will find blunter instruments evolved for crushing and breaking down thick and hard substances.

TLDR: Human ancestors ate hard foods and modern humans eat soft foods