i think the kinky nature of our hair causes it to be unstable and dry since sebaceous oil doesnt travel down our hair shaft the same way as others. if it did i think the quality/strength of most of our hair would be different. but since it doesnt most of us have problems with dry hair that takes a lot of extra time and attention to keep in proper condition.
i also think if we were aware of this as children we could have offset this from the start but it seems as if most women, excluding those that have strong hair that is not common for most of us, dont get into this until adults and the pattern of dry brittle hair has already been established.
so in a way thats genetics since it's why we have such kinky hair. i think it's more about breakage though, and compounded with slower growth rates it seems like our hair just doesn't grow at all. subconsciously thinking the two things are one thing makes short hair a self fulfilling prophecy in how we care for and cut and treat our hair.