Because it is so flexible.
A stiff bristle will get to a knot and just push through. That works for straight hair, but with curly, kinky, or just very thick hair, you can’t just push through because the tension needed to push through will rip out the hair at the scalp first.
You have to slowly work out what hair you can little by little. A flexible brush will pull the loosest strands out, making the overall knot looser, eventually working out the whole tangle.
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