Pixels are commonly square because squares fit together without leaving gaps, have sides of equal length and can be mapped to a grid with two axes – horizontal and vertical. If pixels were circles, there would be gaps when surrounded by neighbouring circles – not ideal for creating smooth images on a screen. Triangles and hexagons fit together too, but their sides follow more than two directions, requiring more processing power to form an image. Square pixels became the norm because there needed to be an industry standard to avoid compatibility issues over different devices. Square pixels stuck due to their simplicity.
Answered by Rik Sargent.