There are two different things coming into play here.

First of all, hair length evolved before long hair did.

Modern humans originally evolved tightly curled hair. Basically like we still see in many African populations. It’s thought that this was an adaptation to protect against the heat of the sun. Basically like an insulated sun hat. The longer this curly hair grew, the more protection our natural hat provided.

As homo-sapiens populations moved further North, this protection was no longer needed - in fact there was the opposite problem, it was cold and rainy.

Greasy straight hair offers an advantage over curly hair in this kind of environment. It acts like a waterproof blanket, preventing the skin beneath getting wet, and drying quickly. Heat is lost through wet skin significantly faster than dry skin, and in situations where energy sources might be hard to come by in winter months, this can be a disadvantage.

We already had hair length sorted, so it was simply a matter of reacquiring the straight hair shape.

Europeans got a leg-up in this regard - Neanderthals appear to have had straight hair, and interbreeding definitely occurred. At this point it’s worth remembering that by the time anatomically modern humans evolved in Africa, earlier hominids were already living all over Africa, Europe and Asia. Neanderthals were very similar to modern humans and shared an extremely close common ancestor.

And this is how, and why, some homo sapiens populations have long straight hair.