The angle of approach is different. Steganography is about hiding a message, you want it to be as undetectable as possible (including using technology). Eventually a transformation may completely destroy the hidden message, but you are supposed to transmit the media as is. The usage is mostly for spying or crime.

Digital Watermarking, you want the "message" to be very robust to transformation. The main usage I know is to help protection against piracy, for example if somebody upload a movie online, you want to put a specific signature related to the location where you sold the movie so you can find the uploader. Eventually the uploader will try to remove the watermarking by either identifying it or by applying some transformation to the movie in hope that it will make the message unrecognizable. So it's a game between industry and the potential tricks that the uploader could apply.

The intersection between the two fields is that they both try to modify the media as little as possible.