Square wasn't the first by any means, and in fact Square's first card readers were unencrypted (cc data sent plain over the audio jack) while most of the industry had moved on to encrypted readers.
What they did was make it cheap, reliable, and iPhone styled. Many of the swiper sources cost upwards of $40/unit and were prone to failure, not to mention black bulky and ugly.
Source: I made an early MVP for a potential Square competitor.
At the start, all Square did on the hardware side was slightly repackage an existing generic swiper. Our MVP swiper was equally small. It was the overall easy onboarding (free decent software, no cost swiper) and marketing that really did it. There were several competing companies who just didn't play the growth game.
What they did was make it cheap, reliable, and iPhone styled. Many of the swiper sources cost upwards of $40/unit and were prone to failure, not to mention black bulky and ugly.
Source: I made an early MVP for a potential Square competitor.