Most of the typical full color resolutions were based on 4:3 ratio - e.g., 640x480, 800x600, etc. - which matched the aspect ratio of broadcast television at the time. Using a 4:3 pixel ratio on a 4:3 display screen meant you had have nominally square pixels. Graphics routines, especially with limited RAM & CPU cycles, are a whole lot easier with square pixels than non-square pixels. Those color graphics cards were typically memory limited in the old days. Memory was expensive, extra memory just for graphics on a card was even more of an extra expense.
However, the original IBM PC had a beautiful 720x350 monochrome 80x25 text mode using a 9x14 font. So it didn't need much memory and it pushed the limits of "ordinary" monitors, but on a real IBM monochrome monitor (or equivalent) it was fantastic. The pixels weren't square, but that really doesn't matter for a text-only display. (OK, the Hercules cards provided full bit-mapped graphics, but that was "business graphics", not playing games (mostly). When color graphics made its way from CGA to EGA to VGA, the 9x14 got bumped up to 9x16 and the total text mode resolution (for 80 cols x 25 rows) went to 720x400. This resolution fully utilized the capabilities of the VGA monitor at 70Hz, and finally brought the horizontal resolution of MDA text to color monitors, for the best of both worlds.