Because BBC Worldwide is a commercial organisation and the BBC (the parent company) is a public service broadcaster. Even though BBC Worldwide is a subsidiary,

Because BBC Worldwide is a commercial organisation and the BBC (the parent company) is a public service broadcaster. Even though BBC Worldwide is a subsidiary, if it started publishing content in the UK (where it has no commercial interest), it would give rise to a conflict of interest. You could argue that if BBC Worldwide were available in the UK without advertising, then it would in effect mean that the parent company was licensing content from a commercial partner.