When you think about it, Sandshrew already has the look of a sandy-brick house in its Kanto form, and they were realizing that they were lacking a solid amount of Ice-types for Gen VII, so Game Freak were like, "Hey, we already have Alolan forms, and Sandshrew already has an igloo pattern, so why not make Sandshrew an igloo?"
Then, "Hold on, guys. Sandslash needs an Alolan form, too." So they were discussin', then they're like, "Since Sandshrew was Ice-type, Sandslash should be as well, and along with that, remember Sandslash's spikes? They can be icicles, which are sharp, pointy, and conical like spikes, and because ice like that can be as tough as steel, let's make Alolan Sandslash a dual Ice-Steel type!"
There's your answer. B-)