The most common theory is that human ancestors ate more plants and needed the extra molars to help grind down the plant material. As human diets changed, we didn't develop as largeand now they aren't quite big enough for wisdom teeth. This makes them a vestigial body part - a part that is no longer really useful for an organism, but has stuck around. We haven't evolved to not have wisdom teeth because the evolution of our jaw happened faster and having the extra wisdom teeth wasn't enough of a disadvantage to our ancestors that those without them were able to pass on their genes significantly more successfully.
Generally, I think the better question for evolution is "why did this evolve?" rather than "why not this?" A lot of times we can't know why something did not evolve because evolution often comes from random mutations so whatever evolutionary trait that happened to pop up is the one we get. However, we can usually make a good guess as to why a particular part of the body was useful.
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