Before the 'revolution' in black hair care began, a lot of hair products specifically targeted towards the care of our hair, were laden with mineral oil a.k.a parrafin liquidum and petroleum oil a.k.a Petrolatum (some of them still are actually). Now while these emollients are not 'bad' in of themselves, the effect they have on our strands in the long run is where the problem lies.
We know now that moisturizing our hair daily with a water based moisturizer and using a deep conditioning treatment weekly is a key factor in making our hair supple, well elasticized and breakage free. When we apply these moisturizers, it's like giving our hair a 'drink of water' - effectively quenching its thirst.
Now think of petroleum jelly - if you smear some on your hand and then pour water on that portion of hand, what happens? The water glides off as the petroleum jelly has created a 'barrier'.
The same thing happens with our hair! When we apply products laden with these substances, they create an un - permeable barrier and any moisture we try to apply subsequently, just sits on top of the strands instead of penetrating through and 'quenching' the hair's thirst. Result? Hair gets dry, brittle and eventually breaks off. This is even more pronounced for those who wash very rarely yet load these products on their hair and scalp very frequently.