He managed to do a decent job with LotR because of working with technological limitations and a good writing team to help adapt the material to film. The fact that the trilogy was as good as it was seems, in retrospect, more of an accident than a certainty.
By the time the Hobbit came around, the movie had a lot of it's planning done by a completely different team (Guillermo del Taco's bunch before he mysteriously abandoned the project). So the leftovers were dumped on Jackson's doorstep after he'd spent the last few years assuming someone else would be at the reins, and leaving him only a small amount of time to get things in motion.
Jackson's main helper for putting the story together was his wife, Fran Walsh, who incidentally was the one who put pressure on the project to include strong female protagonists and minorities. After deciding to include a lot of vague fluff from Tolkien's other writings to make up screen time, Jackson eventually decided on dropping the plans for two movies and bumped it up to three.
Then, like a child in a sweet shop, he excitedly went to town with the new decade's advances in computer animation, eventually deciding to make large portions of almost every scene in green screen with whole environments and characters in CGI, whereas the majority of locations and characters in the Lord of the Rings were real.
Jackson also decided to film the trilogy for a 3D release, which meant that the camera set up was unable to replicate a lot of the perspective tricks he had used in the Lord of the Rings to mess about with scale, things that made the hobbits seem so small and Gandalf so tall without looking jarringly fake.
His studio fucked up the project, his wife fucked up the project, but most of all, he fucked up the project. He was put in a bad position from the start, but managed to do nothing to lessen the damage.
The only decent scene from that entire 10-hour runtime is Bilbo and Gollum's riddles in the dark, which coincidentally was the first scene that was filmed, and which was on a primarily physical set.