Hey Tales, I think this issue doesn't just apply to the book to movie thing.
I think this phenomenon also applies with new interpretations of a franchise.
Like say Kamen Rider
For those who do not know him in this generation, well Kamen Rider was a Japanese SFX hero who is based on a grasshopper created by Shotaro Ishinomori. For a franchise this one last very long time, even until the creator and artist's death in 1998. It's still alive and kicking today.
Here's the first Kamen Rider
The background of this hero is in fact quite dark despite the outdated and sometimes silly looking bug eyed costume. This story was pretty consistent or similar throughout the ages. Howver since the 2000s Kamen Riders of today however deviated from the original concept too much until it seemed to have lost the one thing that made it unique in the first place.
For example.
The first Kamen Rider in 1971. The concept was that the hero was turned into a cyborg. Becos of his lost "humanity", he then decides to prevent the bad guys from turning others into "freaks" like him. Other aspects of his powers include a trusty bike. A finisher which is a powerful Kick and his struggles with himself and his enemies. This is what built the foundations of other incarnations of the same hero.
However, during these years the franchise has moved away from the insect designs which was a just a minor issue but the main aspects of the Kamen Rider is lost. Riders of today do not use their bikes enough or to say no bike stunts. Also the "hero" aspect of Kamen Rider was also lost as Riders of today fight not to save the innocents but for themselves. Also in one incarnation called Den-O, this rider add excessive comedy which doesn't fit in to Kamen Rider franchise.
It's like Batman suddenly having comedic elements.
This is my rant.