The American Board of Dermatology
actually had research as to what makes it curly and why:
Pubic hair is curly because it is genetically made in a flat shape rather than a round shape. Perfectly round hair, such as those on scalps of Native Americans, is perfectly straight and has no tendency to curl. However, ribbonlike hair on the scalps of blacks is seen to curl because of its oval-like construction. The same is true of pubic hair... --Dr. Joseph P. Bark
But what function does the curliness serve? Well, some dermatologists speculate that if it grew straight and stiff, it would rub against adjacent areas and cause discomfort. It also grows much slower, so you don't trip on it I guess.
It's not always curly. Early in puberty it's soft and straight. Most Asians have sparser and straighter pubic hair than that of whites or blacks.
--quote taken from Do Penguins have Knees? by David Feldman