Why does the wavelength in curly hair differ?
My hypothesis when the curls are left ungroomed they will form knots. These knots form a barrier that works similar to woven wool. Unlike strait hair the mat of knots (dreadlocks) collect dirt and form denser material that insulates against direct light and mosquitoes better than strait material. The wave length does play a part as well. The smaller the wavelength the tighter the knot. The tightest knots are commonly found in places that have the most sun light per year.

Strait hair tends to keep heat in more and ice builds up less on it.