Early two-tined forks worked well for holding meat being cut but were not useful for scooping up peas or other loose food. The bulbous tip of the knife blade evolved to provide an efficient means of conveying food to the mouth, with the curve of the blade reducing the amount of wrist contortion needed to use the utensil thus.
With the introduction of three- and four-tined forks, the latter sometimes called “split spoons,” it was no longer necessary or fashionable to use the knife as a food scoop, and so its bulbous curved blade reverted to more easily manufactured shapes. However, habit and custom persisted at the dinner table, and the functionally inefficient knife was used throughout the nineteenth century by less refined diners for putting food in the mouth.
From: The Evolution of Useful Things by Henry Petroski