The primary reason is because operators are mainly competing and not complimenting. They have no interest in telling you about other options which are not theirs. In much the same way Tesco don't tell you it's cheaper to buy something from Asda, for example.

Also, rail is heavily franchised, so there is really no competition there and it's probably within their agreements that they jointly promote things.

Bus is different, less franchising and more commercial. Competition Law plays a hugely significant part of what bus operators are allowed to do, say and share.