LOL, based on the experiences in my family, I would say it is more a factor of genetics than anything else. While I have known some Haitian women with beautiful heads of long, thick hair, all the women in my family (including my mom, who had 3B/C hair) have never had hair past BSL.
Also what helps is the Haitian tradition of non-manipulation and no chemical processing, at least among older women. My mom, who has never had a relaxer in her life, has the longest hair in the best condition, although it has always been fine and somewhat thin, though BSL. My older sister, who has also never relaxed, follows closely behind with her extremely thick, 4a/b BSL hair. My baby sis, who has been natural for almost 3 years now, came back from nearly destroying the edges on her 3C/4/A hair due to over processing and tight ponytails. She did the BC and now sports a pretty, tightly spiraled natural that is APL, almost BSL.
As for me, I have fried, dyed and laid to the side my fine, fragile 4B hair from the time I was 15, and could never make it past shoulder length throughout my teens and twenties. Even when I went natural for a few years in my thirties and made it to APL, dyeing it bleach blonde would eradicate any progress I made in length. Now I'm 42, relaxed with a solid regimen, and my hair is at APL. Hoping to kiss BSL in Jan, which is great for my family, but not so much compared to my other Haitian friends.