I'm sure this has to do with the licensing agreement between Spotify and Hulu. The bundle links each Hulu account to 1 premium Spotify membership. If they allowed the bundle on the Family plan, each user would get Hulu access for free which amounts up to a $30 value add if you have the max 5 family members. All for $15 /month.
I don't think it's as simple as saying "all linked accounts get 1 Hulu login".