Perhaps the simplest explanation is the natural development of polearms. Polearms are intended as mass weapons, to be used not just by individual warriors, but by formations of soldiers
together on field battles and not for dueling. When fighting in close order, two-handed cut-and-thrust weapons, such as halberds and glaives, are much more efficient than mere spears or
swords because of their versatility compared to spears and longer reach compared to swords. Fighting in massed formation does not require similar individual weapon-handling skills as
required by a skilled swordsman.