PNG has been updated for the first time in 22 years — new spec supports HDR and animation
(www.tomshardware.com)
The current situation with megabytes of JavaScript is pretty bad, but at the time, there was still a fair bit of dialup active, and mobile web was just starting to be a thing - on EDGE and barely 3G. It would take minutes to load.
Also, Steve Jobs had it in for Flash and that’s what ultimately killed it off, I think.