Why? This is much more similar to Tolkien's description (from the Silmilarion, since he never appeared in the books except for a very brief moment in the end as a shadowy avatar right before dying). The design in Jackson's trilogy was somewhat rather similar to Melkor. Tolkien based Sauron and Melkor on the devil himself. So he intended to have spikes sticking out of the helmet to make it look more devious.
Well, yeah. Tolkien based Sauron on the devil himself. Since Tolkien was a Catholic and read the Bible a lot. He saw the devil as a tall being with horns. The movie version of him is somewhat similar to Morgoth's helmet.