Swords were particularly rare and expensive in the early middle ages. There were only certain parts of Europe known for their swordsmithing, and they were all controlled by different feudal powers; Frankish swords were high quality and prized by the Norse, for instance. It was also more difficult to learn to use a sword effectively as compared to a spear or an axe.

This kind of specialized training, along with mounted combat, the use of the lance and horsemanship in general, gradually became the province of the knightly and noble classes. Swords became a status symbol and, in some cultures of the medieval period, only knights and noblemen were allowed to carry them (especially during peacetime).

As the middle ages progressed, swords become more and more commonplace, just as the province of warfare begins to expand from the nobility to include professional men-at-arms of lower social rank.

Comparatively, the Roman Republic/Principate/Dominate is a completely different beast. Not only is it a massive polity stretching over the entire Mediterranean basin, it is also a military juggernaut that relies on its legions to continuously conquer new territories. Prior to the Nerva-Antonine dynasty, the idea of 'fixed borders' would have been not only ridiculous but sacreligious as well; the Romans believed they had been granted infinite imperium by the gods. Each victory brings more land, more minerals, more slaves; these resources are exploited and become fuel for the furnace of conquest.

Rome had centralized administration, tax collection, heavy industry, vast natural resources, a state-subsidized, professional standing army and labor that was extremely cost-effective (to put it mildly). States with all of that, on the level of efficiency that the Romans had achieved, would not be seen again in Western Europe until the early modern period. Those are all contributing factors to why Rome was able and willing to equip their legions as they did.