The answer is prior chemical over-processing. It's what raises and damages the cuticle, which is why the hair soaks up product so fast.
A raised, damaged cuticle lets liquid in quickly but also lets moisture and color pigment escape just as fast, which is the real problem with over-porous hair. Diet, hormones, and scalp conditions can affect hair health generally, but they aren't the direct cause of high porosity itself.
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