Is there a reason that Kyoko is not pictured in some of the series' more prominent artwork? As part of the story's main set of magical girls, and a character important to the plot, I would expect her to be featured with the rest of the quintet. I consider this piece
to be the anime's "official" artwork, so (barring whether or not that is a valid assumption on my part) why is a character as important as Kyoko not featured in it?
The image above was used for Netflix1 in the past and is currently used in Crunchyroll's listing as well. The current (Dec 13 2014) image used by Netflix is apparently a different one but nonetheless still does not feature Kyoko.
Another instance of this "phenomenon" can be seen in the first volume of the manga:
though an easy explanation for this one could be that
Kyoko doesn't appear until the very end of the first volume, and/so her character is not significant to the first part whatsoever.
So my question is this:
Why doesn't Kyoko appear in the anime's more iconic or official artwork, or are my assumption(s) about the artwork incorrect?
EDIT: More cases
1This is the image I remember being used when I saw it on Netflix a while back, but I'm not sure if it's been updated/changed since, because I don't use the service anymore.