> PNG rejected the APNG proposal multiple times ( Why ? )
Because it is not a still image, and therefore should have a different file signature, MIME type, and file name extension. It is simply out of scope for PNG. When designing PNG as a replacement for GIF, they very deliberately and decidedly dropped the animation feature. Implementing APNG as an extension of PNG confuses categories that should not be confused.
The main goal was to create a patent-free replacement for GIF, with a number of improvements for its main uses. GIF animation was not used much at that moment.
Still, it's not like it was impossible to make some last minute changes, while PNG was still relatively young. See https://boutell.com/innards/apng.html
You have to admit it's a little funny that if PNG's one goal in life was to replace GIF, they chose to not support the one thing GIF is used for today.
The popularity of animated GIFs, or the use at any scale of them at all really, only came after PNG was finalized.
The design of PNG started in January 1995 and PNG 1.0 was published in October 1996. Netscape Navigator added animated GIF support in September 1995, and if I remember correctly, it wasn't immediately used at large scale. Anyway, by that time, the PNG team had IIRC already decided to put animation in a yet-to-be-designed MNG.
Because it is not a still image, and therefore should have a different file signature, MIME type, and file name extension. It is simply out of scope for PNG. When designing PNG as a replacement for GIF, they very deliberately and decidedly dropped the animation feature. Implementing APNG as an extension of PNG confuses categories that should not be confused.