I don’t really know the reason, but it would be cool if that screen included a reason for defederation alongside the name of every defederated instance.
That said, wasn’t Hexbear using a Lemmy fork that split off really early and then added lots of features of their own, making it particularly incompatible with the rest of the Fediverse? I read somewhere that federating with Hexbear was not possible at the moment and that it’s unclear whether it will ever be possible.