(a) Creative leaders (producer, director, writer) hired for these projects chafe at having anyone tell them what to do (and this especially includes the source material!).
(b) Creative leaders hired for these projects are typically not themselves fans, and do not elevate anyone who is a fan to a position of authority, and so lack even an awareness of the material. And critically, an awareness of what current fans enjoyed in it. Which is sometimes a feeling or deeper than just "these specific characters or places."
(c) As big budget projects, and costly rights acquisitions to start with, there are powerful committees put in place to keep creative on the rails, who are even more (a) & (b) themselves.
As a result, there is literally no one in the room with a powerful voice who understands why so many people liked this thing.
Say what you want about recent Marvel & Star Wars, but Feige/Favreau/Filoni seem to have a better sense and balance of "Why did people like this thing originally?" and "What will attract people who aren't fans?"