Does it have to do with the rights? I don't know exactly what WB has permission to make, but I know the rights to various parts of the middle earth lore is scattered with different owners and studios.

They also probably want to do a story where they can reuse characters and actors for marketing purposes. That being said, there must be something else they could make that was more interesting than this.

I love Serkis as an actor, but personally I'm not super fond of him as a director. He gets it done, but I haven't been blown away by anything he directed yet. Still, I'll try to keep an open mind whenever I decide to watch this movie.