Why? He has no linguistic expertise, and he didn't have the perspective of the format's popularity when he made that decree. And his decision was based on intentionally infringing on copyright. And it intentionally goes against the intuitive pronunciation. And the term "gif" now even refers to files that aren't even .gif - it's way past him.

This may sound harsh, and I want to acknowledge that he did something really awesome - but the Jif pronunciation will not survive once he, as a person, is forgotten. But the format will. It's not his anymore.