It's ghost for the same reasons Decidueye are, ghost and dark types when not associated with the spiritual or the malevolent are now associated with stealth and the ability to obscure, manipulate or remove themselves from sight, as well as phasing into and out of reality to deceive occasionally.
It should be noted, however, that this dragging in of ghost type to the general stealth aesthetic in rather than just as tricksters pranking your vision or dimensional being hopping planes of existence as it had been before is a newer convention, around the gen 5/6 mark. If it seems weird to you that ghost is tied to the idea stealth now, it's because you grew up with older generations, when this was more focal in dark's aesthetic lineup rather than both dark and ghost's.
Welcome to a changing world, Pokemon will continue to make more decisions you don't like seeing in it as they progress and new talent (and new "talent") are phased in while old are phased out. One too many people being phased in thought the stealth thing would work for ghosts too, so now we're getting it as such, and will likely get more in the future.
