I suspect over the course of his life Tolkien became more and more frustrated with technology, and removed most overt traces of it to bring his stories closer to what he read in Medieval folk lore.
There's also the hint, at least from the movies, that Sauron and Sarumon could produce an unlimited supply slave labor.
"True magic" seems to stem from intimate knowledge of the more arcane and foundational aspects of Arda. The Ainur wield it instinctively, and only the most ancient Elves seem to be able to learn it. But also they struggle to use it on scale, which was one of the reasons the Rings of Power were forged. In the Third Age, advanced magic seems to only be used via the Rings of Power, and dissipates for good after the destruction of the One Ring.