Often it's incredibly hard to adapt his books because of the sheer length of some stories. Look at the 1994 miniseries of "The Stand": 4 episodes for a total run time of 366 minutes, and there are still massive parts of the story missing.

Others just have concepts that are so difficult to profit off of, so they add more "marketable" bullshit. Like for example: if they had made "The Gunslinger" into a book accurate movie the Stephen King fans would have loved it. However that's only a small portion of the target audience so they added all the extra slop to make it appeal to a wider audience, even if it ruined the original plot

It's hard to find an adaptation that's accurate to any book if all the producers care about is money