I've never heard of APNG in my life before reading this thread. I've always liked PNG's more than GIF files, so an animated PNG would be great if they supported it more.
The honest answer of why Graal switched away from .GIFS I think is because of this:
At the end of December 1994, CompuServe Inc. and Unisys Corporation announced to the public that developers would have to pay a license fee in order to continue to use technology patented by Unisys in certain categories of software supporting the GIF format.
There was a usage tax back then. I remember working on 2k1 and other servers and Globals popping in scanning our files for .GIF files and making us remove them. But as of 2004 these usage taxes and patents have expired.