I know that in JPG and TIFF images GPS location is (can be) stored in the Exif data. Are there any other formats that provide GPS location in metadata? I looked through the PNG specification and couldn't find anything on it.
Any image format that can handle metadata can store GPS locations in it, from what I can tell PNG can handle metadata, but the problem is that no one has bothered with storing that kind of data for PNG simply because it is not a common format used in cameras or location aware devices.
As to why it's never had any real support for adding in location-aware-ness then that's a question for the developers. At a guess I'd say it's probably because PNG has only really gained popularity as a free replacement for GIF images and JPEG is far better for it's high compression and reasonable image quality when used with modern high-megapixel digital cameras. A PNG from a 5MP or greater camera would be huge, while a JPEG is a reasonable size.
I just doubt there's any real drive to crowbar in a feature that isn't really needed.