Why option a story and then change it?

If you are watching Miss Fisher Murder Mysteries, or reading the books, there are spoilers ahead.  Although we're talking something that was new a year ago, so spoilers should be expected.

Netflix recommended Australia's Miss Fisher Murder Mysteries to me and I loved all three seasons of the series.  Smart, clever characters, great stories.  I was bereft when I ran out of episodes.

And would still love to see a season four, but it's been over a year, so I think that's not likely at this point.

Based on the series, I decided to read the books by Kerry Greenwood.  Equally smart, detailed characters and clever stories.

But very different from the TV series.

In the series there is a whole subplot about Phryne's murdered younger sister.  Didn't happen in the book.  There is also a subplot involving Phryne's ne'er do well father showing up on her doorstep.  Also, didn't happen in the book.

I understand there were 20 books and 34 TV episodes so some stories had to be created.  But why change a fundamental part of a book series that you obviously felt was worthy of being filmed?

I can understand the change of Detective Jack Robinson from compatriot to potential love interest from a production standpoint.  Keep that "will they won't they" tension building.

The loss of one servant (Mrs. Butler) and one adopted daughter (Ruth) can be explained by casting costs.  And maybe the expansion of the roles of Bert and Ces, Dr. Mac, and Hugh lessened the need for supplemental characters.

I still don't know why Aunt Prudence was added.  She is given only a passing mention in the books. Although she was a delightful character in her episodes.

Often the cause and the murderer differed drastically from book to series.

Why keep the title of the books and completely change the story?  I mean, the only thing they kept from Dead Man's Chest was the location and the rumor of pirate treasure.

In the book, the house was a rental and was empty when Phryne, her TWO daughters, and Dot arrived.  The servants were later to be discovered hiding from rum runners. It was a story about intimidation, aspiration, and courage.

In the episode, the servants were murdered. Phryne, Jane, Dot, and Aunt Prudence were to be guests at the home of a woman who, it was discovered, had a serious drinking problem. This one was a story about addiction, obsession, and desperation.

Both worthy stories, but why give the episode the same name as the book?  The culprits were different, the entire book subplot of the movie was abandoned as was Phryne's fake treasure hunt used to lure the villains.

Make new stories with the characters.  Don't remake the existing ones.