OK. I'll bite. First of all, the fact that another wonder may also not have the ideal icon, does not make that it's right for Kremlin to be depicted by the wrong icon. So what? Another wrong doesn't make this right.
Secondly, Chitchen Itza is represented by the Temple of Kukulkan, which is arguably the center of Chitchen Itza and its most prominent building. So they selected a prominent building inside Chitchen Itza to represent the whole. This is quite logical since it's iconic for the site and how else would you represent a whole historical complex in a meaningful miniature?
If they had selected a piece of Kremlin wall to represent the Kremlin, I'm sure Motor-Ola wouldn't have created this thread.