It's a bit of a complicated issue, but I would summarize as:
HBO exists as a broadcaster dedicated to showing 'adult' programming which aims to reflect some of the darker elements of life - whether through comedy or drama. It has a large subscription base that want to watch programmes on this basis (ie, with the sex, violence and bad language left in).
Game of thrones was bought and produced by HBO specifically so that the final version could accurately reflect the books. And many of the people that watch the show like it on the basis that it does contain these 'real' elements. It would seem absurd that just because the show has now reached a wider audience that it should change to accommodate them. I know that this kind of thing happens all the time in commercial industries however not for HBO programmes.
If you want the show to be Merlin why not just watch Merlin?