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".
I’m not sure about other Spotify users, but for me I’ve only got two people on the family plan, myself and my sister since it’s cheaper that way. So it still is kind of annoying that the family plan doesn’t get included in the promotions that Spotify has going on.