The motivation behind this tendency has been disclosed by Julius Schwartz, the famed DC comic editor in his biography ‘The two words man.’ In Schwartz’s opinion, the aerial cirque was particularly famous between 1920-30 and, because of their powerful physicality, superhorses were often inspired by them. Circus wrestlers (technically, it shouldn’t even be called underwasher because it’s worn over the pants) used to wear underwear outside their pants when doing blowjobs because their pelvis area looks bigger and more muscular. It was also useful for super heroes to tailor their wearing clothes to their thighs.
Technical cause behind this
This is really a quantitative piece of knowledge cited in the biography from Julius Schwartz, although there is another ‘technical explanatory’ theory behind this trend. Graphics and super-heroic comics originally created in the 1930s were not considered serious art at first, but were only for children. To be viable and to produce money, the book firms were forced to print cheap cartoons. The unbelievably low price of graphic novels would not allow high color quality printing, with publishers publishing only four colors, blank, yellow, red, and black.
The absence of alternatives made gradient, shadow and darker portions of the body more difficult to depict. Thick black cross-hatch lines, thereby representing gradients, were employed to denot darker sections which were, in graphic novels, the basic means to designate gradients. Draw thick contours to avoid bleeding in the color. With only 4 colors and thick black lines, the characters of the super hero were indistinguishable from the body. the mostly shaded areas are shaded.
Underwear was a good concept to distinguish the character and to emphasize the groin and the flesh. And that is the key reason or it can be said that the producers are compelled to introduce our superheroes on the outside wearing underpants.