Why is Kyoko not present in Madoka Magica's official artwork? 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


Netflix listing, Dec 13 2014
Older Netflix listing screenshot ... (source)




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.
There are many other official artwork for the series. That is the cover of the DVD box set and like you said, the other image where Kyoko is not included is the cover for the first volume of the manga. 



This is probably also the reason why Netflix used that artwork as its listing.

I think iconic is subjective in this case. Someone more familiar with the movies will think differently, as you'll see later, but there are a lot of other official artwork where only two magical girls are featured at a time:



I do think that the reason why she's not included in that artwork is because she's not involved in the first three episodes (there's 12 episodes and the set contains 3 discs, assuming the first disc only contains the first three episodes). At least Sayaka is introduced in the first episode, which is presumably the first episode the viewer will watch after purchasing the discs set. Bear in mind that the cover for the discs set differs from region to region. 

Finally, the artwork for the disc for the first movie, as well as the movie posters, includes all of the magical girls:



In the first movie all of the five characters are introduced and played significant roles. So this theory is not that far-fetched.