My answer is...it depends.
Sometimes the author was the one with the ideas, but needed help with the visual expression of those ideas, so they hire or pair with an artist/illustrator. If not for the author, the deck would not exist. They deserve the credit for creating the deck.
Sometimes an artist has the ideas and creates the deck, but needs help to verbalize what they have expressed in their art, and they hire or pair with a writer. Then the artist deserves most of the credit.
Sometimes it is truly collaborative and both deserve the credit.
Most of the time, it's not easy to determine which scenario applies to a particular deck. An exception is the New Orleans Voodoo Tarot. The book's cover page states "Concept by Louis Martinié and Sallie Ann Glassman, Text by Louis Martinié, Artwork by Sallie Ann Glassman". I think the 'Concept by' is really the key, and I wish more tarot sets would be as forthright.