I think it's because it's probably the first thing that has the true potential to bring AI and its potential to mainstream awareness. They may not want that at this point in time — gives them time to keep working away (mostly) in private. It may or may not be a good idea though; somebody's going to do it regardless if they don't.

If a model was released that enabled anyone to create beautiful music in any genre effortlessly (or even easily,) the shift in public perception would be absolutely tectonic. Everyone would be talking about artificial intelligence. Image generators caused a small stir. This would incur societal level awareness of what's happening.