As of December 2020, all major browsers support JPEG, but not all fully support WebP [https://caniuse.com/webp]. Users of browsers without WebP support won't see the images when visiting the page, so this might be why your blog platform doesn't accept WebP files.
Another downside: if someone downloads an image, the downloaded file will still have the ".jpg" extension despite being WebP format. Many programs depend on file extensions to know how to open it. They will try to decode it as JPEG, but fail, since the file is WebP.