The original impetus that led to MNG was animated GIF. While GIF was also designed as extensible, GIF practically only had a handful number of extensions and wasn't that complex. MNG in comparison was essentially a binary version of SVG as I have described, except that it had no vector graphics and can more accurately be described as a binary slideshow format [1]. No one asked for this! The PNG development group forgot its goal and made a much bloated specification. As a result libmng, while working, was a quite big library to be incorporated into Gecko and ultimately rejected in favor of APNG which only required a small patch to libpng. It is amusing that the original PNG specification had chunks corresponding to GIF extensions including animation one; those chunks might have been widespread in the alternate universe!
If the group's goal was indeed a slideshow format, it should have been two separate formats, one for multi-part images and one for animations and object transformations. The former is necessary because one aspect of GIF was that a difference from the previous frame can be encoded, so this "Delta-PNG" format should have been made into the mainline PNG specification as an option. (These additional images could have been used as previews, for example.) And anything else should be a simple textual format that refers to external files. MNG instead had chunks for JPEG images---came with its standalone sister format called JNG---, which is absurd when you think about it---why should there be JNG when JFIF and Exif already exist? The modern SVG happens to be perfectly usable as this second format, and it is fine being textual.
[1] If you can't believe what I've said, see the table of contents of the actual specification: http://www.libpng.org/pub/mng/spec/