The only reason i could think of as to why the spear isnt already a 1 slot weapon is because of the 4-directional attacks. With roughly the same stats as the military fork, plus 4 directions it beats the fork out in combat without a shield. The only reason to pick the fork is the fact it only takes 1 slot.
Dont get me wrong i think itd be nice to have a 1slot spear with a little more range than the shortened spear, but making it 1 slot would make the military fork useless. A way it could work, however, would be to buff the damage on the fork to compensate for the overhand/stab limitation (especially with the nerf to them), then give the spear 1 slot. If that was done, both of the weapons would have thier own niche.