Because it sure as hell doesn't look light, at least not from the one picture we've seen of it.

In morrowind it was in the medium category. Probably it will be placed in heavy armor category.

in morrowind there was a medium(?) version and a heavy version I think, and it looks like there will be multiple types from the screenshots

Hard to say, really.

I'd put it as light armor, just because of how its made (crushed bone mixed with resin to form plates, mounted on a cloth undersuit.) But we won't know until Dragon Born is released.

Well Orcish was medium in Morrowind, and that was made heavy in later games. So I imagine Bonemold will be the same.