Printing technology and cost.
In the early 1930’s and 40’s printing of comics came in two forms, black and white and 4 color, this is also where we get the term 4 color hero. In general comic books were intended to be as cheap as possible so they often used the lowest grades of paper and the fastest and cheapest printing methods. The color printing was usually only reserved for the cover page of a comic because it was a costly process that required the ink to be applied in 4 separate stages, one for each color. The problem became that when doing this the machines had to run at a very high rate of speed to produce enough comics and they would eventually become misaligned and need constant adjustment. This is why we see comics from this era onward with the colors bleeding outside of the lines.
Esthetics and the superhero persona.
Working within the limitations I described above, comic book artists took great strides to make powerful and lasting impressions. Right or wrong and consciously or not, this led to emphasizing hyper masculine or hyper feminine character traits to make the characters seem larger than life on such a simple format. We often see color changes or divisions at the waist, groin, feet, hands and chest. This allows the characters to have certain “attributes” stand out.
One of the many reasons why the Internet is such an amazing thing is the fact that you can get answers and discussions on these kind of questions on Quora.