Because browsers have an enormous API/file support surface as is, and each new image format can be a significant amount of code to support essentially forever.

Because browsers have an enormous API/file support surface as is, and each new image format can be a significant amount of code to support essentially forever. Webp now is supported by pretty much every browser, but a lot of image viewers don't support the format yet. There has to be a certain amount of momentum to justify supporting a new image format.