Froakie & Greninja

Artistic interpretations

I have seen rationale shared by some folks online that the reason the author used “artistic interpretations” of the Froakie and Greninja may be a licensing or copyright issue. However, two of the actual cards that are stated to be in the treasure are photographed and included. Why couldn’t the author have just purchased and photographed a Greninja and Froakie card and displayed them under the same usage rights that made him and the publisher feel comfortable enough to use the Charizard images as they did?