I think it has to do with the curl type and curl texture. When a curly's hair is coarse, it wont look silky, and therefore giving away that your not a natural straighty. Also, if you have a tighter curl pattern, like 3bs and down, your hair wont stay perfectly straight for long, that is if you even get it perfect at all.

2c/3a
Growing out hair to a little above bsl
pw: ringlets