This wasn't always the case. The fork actually started out in Western Europe as an agricultural tool, eventually evolving to become a kitchenware in the 1500s. Back then, early versions only contained two tines. But according to Bill Bryson, author of the book At Home, two-tined forks caused a lot of pain to diners who accidentally jarred themselves, perhaps after one too many tipples.
To rectify this problem, fork makers experimented with a greater number of tines, but ultimately settled on four as the most comfortable for diners.