In the days before gunpowder- heck, even the days before the wheel- the spear has been a widely-used weapon for no other fact than they are easily and cheaply constructed, and rather simple to use. There are a number of permutations that have developed over time- the trident, the yari, the naginata, the lance, and a number of other similar weapons, but the design has always remained somewhat stable- a pointed end (of flint, copper, bronze, iron, steel, or the sharpened wood itself) on the end of a long pole.

For all its simplicity, the spear is a versitile weapon. The invincible Greek phalanx, using spears that were sixteen feet long, was considered the invulnerable unit of its time simply because it was extremely difficult to breach them. Lances were used by knights on horseback to reach out and touch someone whom their sword couldn't reach, usually some hapless infantryman or another knight. Pikes are extrodinary anti-cavalry weapons- brace the end against the ground, and skewer the horse. The horseman's saber isn't quite so useful now when he's outnumbered at the front line, ja?

Of course, the fact that spears are inexpensive and easily manufactured also makes them good for arming the peasantry. While there are schools that teach serious skill with a polearm, the spear is an easy weapon to use for the unskilled- the extent of the common man's instruction with the weapon is often limited to "pointy end goes in bad guy," and "kill the horse and you might live."