When png was released, it was unsupported by the majority of browsers (and is still not supported by everything mind you) but didn’t have a fallback to a more widely adopted format. It was finalized 9 years after gif, which admittedly is a third of the gap between now and png finalization.

Fallback support isn’t needed. It never has been before, why would it suddenly be needed now? Servers are more than capable of checking the browser on request and serving a different format based on that. They’ve been capable of doing that for decades, and the effort that goes into it is virtually non existent.