In an unarmored context, which applied very often throughout history, the spear is easier to use and especially lighter, which makes it a better and more nimble weapon. Spears can also be much longer than heavier pole arms whilst remaining usable, keeping the danger further away from the user

The speed at which one can move a spear tip is impressive and getting stabbed by it has large stopping power. The spear can also parry attacks in a large sweeping area, which makes it hard for anything else than another spear to get through

The more complex pole weapons start to shine in an armored context, where stabbing someone at the end of your long pointy stick becomes harder. Then, the hook-y, chop-y and spike-y bits of the halberd can really help tackle the armor