Why do different hair types exist?

(When an ethnicity/race is mentioned, I'm referring to the norm, not the absolute. Example: "Africans have curly hair, Europeans do not." This is the norm, I know many exceptions exist. But for this page, the majority is all I'm working with.)
In short: no one's entirely sure why.
Thicker hair is, however, the dominant gene. This gives an explanation as to why nearly everyone with blood mostly from Africa--an entirely isolated continent up until a few hundred years ago--has the signature thick Type 4 African curls and coils. However, it provides a loose explanation of why mixed children tend to be thicker Type 2s and 3s.
Some research suggests that the curls first came about to protect Africans' scalps from the sun. Just like how extra melanin was pumped as skin protection, hair may have curled and kinked to cover your head. This, to me, explains why so many Africans have really thick hair. The thicker it is, the better protection it served, so coils are what spread across the continent. Also, it also explains why body hair normally remains pin-straight no matter how kinky the head hair is. However, my question would then turn to India. Unfortunately, I couldn't find anything touching on why many Indians' hair tends to be much straighter than Africans. Instead of coiling and standing up, the only resemblance their hair has to Africans' is a bit of thickness, which still flails in comparison to many Type 4s. Despite being just as hot as Africa, if not hotter, their hair resembles Europeans' hair more in terms of texture and curls. They got the melanin adaptation for the sun, so why not the curls also? My guess would be that interaction with nearby and much paler countries kept Indian hair on the straighter side.

Go here for more information on the specific gene that influences hair texture:  https://genetics.thetech.org/ask/ask107