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.