Ultimately, it's a practical issue, not an artistic one. No studio in their right mind is going to worry about contradicting some old tie-in novels when it comes to making a Major Motion Picture or TV series which will be seen by millions more people than have ever read the books.
We're talking whole different orders of magnitude here. Movies have audiences of millions . . or maybe even billions. Tie-in novels are read by tens of thousands, tops.
And I wouldn't take offense if a new movie contradicted one of my books. Hell, my novelization of UNDERWORLD: RISE OF THE LYCANS completely contradicted my earlier novel, UNDERWORLD: BLOOD ENEMY. Why? Because the new movie continuity took priority.
That's just the way it goes. The movies and TV episodes always trump the books when it comes to continuity. They're the dog; we're the tail.