In most cases, unless natural afro-textured hair is not combed (i.e. styled or grown into dreadlocks), its unique shape itself often limits the length that the hair can reach because the act of combing and brushing tends to break the ends of the hair; resulting in what is effectively a "natural haircut" each time it is styled. In other words, due to conventional grooming practices, loose natural afro-hair, upon reaching a certain length (which varies by the tightness of the coil), reaches a "steady state" such that it does not appear longer despite continual new growth.
Source: Khumalo, NP "African hair length in a school population: a clue to disease pathogenesis?" Journal of Cosmetic Dermatology. Sep 6(3),144-51. 2007