For the most part, low-quality steel. As until almost the Renaissance, good-quality steel production was impossible for the small-forge smithy to produce, swords were made with relatively low-carbon-content steel with a considerable impurity content, which tends to make them much more brittle than other steel alloys.
In certain parts of the Middle East, more advanced steel making allowed the production of "Damascus Steel", which had a lower impurity percentage, and also a unique forging process (one that has since been lost). Such blades were considerably stronger (and could hold an edge longer) than similar-era European swords.
With the improvement of steelworking in the mid-Renaissance period, swords became made with higher-carbon steel, and the level of impurities dropped radically, producing a much more flexible and strong metal.