There's also wanting to have separate molds for each line so that they can be molded in different materials, or in entirely different factories, at the same time.
Bones Black material is more solid once cast. It still has some flex to it on thinner elements, but not nearly as much as original Bones. Swords and such won't be bent out of shape as on Bones, but the new material also won't be re-poseable by just soaking it in hot water. That rigidity and color change also makes details sharper and easier to pick out. It's less resistant to damage than original Bones, but not as likely to snap when dropped as typical plastic minis and not prone to shattering when dropped as resin minis can be. But avoid dropping or chucking it on concrete anyways. Really sharp external details (pointy ears, ect) will be damaged if it takes a hard fall, but I expect pretty much everything except the super bendy Bones would be to some extent.