APNG is a funny, political beast. It basically evolved after Mozilla decided to nuke MNG support to make Firefox slimmer and then needed to re-invent a transparency-capable animation format for browser "throbbers". There were calls to replace libpng with libmng which purported to support both PNG and MNG at the same binary size, but those never panned out.
IIRC, individuals in the PNG group took the whole thing as a slight and then refused to incorporate it into the official spec, leaving it as a bit of an orphan. The Chrome guys never bothered implementing it and quietly worked on WebP.
I suspect that part of the reason APNG never received widespread adoption was bad blood from MNG never getting proper attention. While MNG was kind of a crappy spec in retrospect, it was still way better than an animated GIF.
Unfortunately JNG was also a casualty in this whole war -- a format that could have been both lossless and supported proper alpha transparency.