Q: Why doesn't PNG support animation?
A: The answer is either "it does" or "because it's a bad idea," depending on your perspective.
The animation-supporting version of PNG is called MNG, for Multiple-image Network Graphics. For most practical purposes, MNG is PNG, just with a different filename extension, a slightly different file signature, and the bare minimum of internal changes necessary to support animations (and other forms of multi-image files). It has exactly the same chunk structure as PNG and even shares most of the same chunks; the major difference is the addition of new chunk types to support features such as looping, clipping, and so on.