I expect that the unslotted is also slanted unless it's Fender-style!
It's an aesthetic and financial decision. It takes a bit more time to make a slotted peghead, and therefore it costs more. It's a bit more difficult to change strings on a slot-head, but they're pretty. Explanations that go into string break angle behind the nut are fairly bogus because the builder can make the angle anything you like, slotted or paddle...