They failed to understand why the books are so legendary
First let me say. The series was not that bad. The problem I have is the show failed to understand what made the Musketeer books to great. It was Dumas use of words and humour that came naturally. Now if you read all the books then you know that there were very dark parts to all the books. But the humour and adventure and swash buckling was jolly good fun most of the times
I realize that most of the stories have been told to a point--but there was a lot in the books they could have used.
For me the series failed to capitalize on the energy of the books and the elements of why for over 100 years Dumas is still so widely read and loved