I think that it's worth noting that Spikes was, when it was first introduced in Generation II, was the signature move of Pineco and Forretress. Neither are Ground types, and it's the only move that they both learn by level up that isn't Normal type, as well as the only Ground type move that they can learn, including TMs and egg moves. Furthermore, the only ground type Pokemon who can learn Spikes at all is Sandslash, which is only as of Generation 8. Also, considering that the spikes appearing in the move animation appear the be based on caltrops (https://en.wikipedia.org/wiki/Caltrop
). With all of this in mind, it makes much more sense for Spikes to be a Steel type move, especially when you take into account the fact that Forretress is a Steel type Pokemon, and there are multiple other Steel types that are able to learn Spikes. Normal type would also make a lot of sense, considering that there is a very large variety of differently typed Pokemon that can learn Spikes. The only reason I can think of that there would be for making Spikes a Ground type move is that the spikes go on the ground. But in that case, why not just make it Normal type? I can see it not being Steel type because Pineco, which is not a Steel type, can also learn it, or maybe the move was created before they decided they were going to add the Steel type, and Ground just seemed like the type that fit the move best.