By Ariel Williams, Dreamer, Artist, and Writer
This is actually an interesting question, but the answer can be explained very easily so I will add some images to spice it up. The trend the question describes we see not just in Superman but in many comic book heroes with their origins from the earliest days of comic books from the 1930s and 1940s onward.
Printing technology and cost.
In the early 1930s and 40s, printing of comics came in two forms, black and white and four color, this is also where we get the term four 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 four 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.
Due to these minor imperfections in the process itself, the comics were produced with sharp clean edges defined by hard black, and often, the layouts would be done so that objects could be painted a single color. These restrictions and a lack of a proper gray constrained the art style to fit within the technology of the day. The methods they used to overcome this came in using either pointillism as the image above or hard solid colors as below.
(High magnification scans of comic book details from 4CP | Four Color Process (posterous.com) )
Aesthetics 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.
Which one looks more "heroic"?
The center option almost seems to have neutered Superman with its lack of definition. While option three might be acceptable in this panel, in some poses or in very small panels in the comics, his legs might overlap the groin area and the entire pose might lose definition. You literally might not be able to tell his leg from his a-hole. Also, if the comic specifically defined his "package," it might offend some 1940s' sensibilities.
Even characters that wore only a single color often had detail lines outlining the pelvis from the rest of the body so their features could easily be made out on small panels.
Here, we can see what looks like "undies" even on human torch and Mr. Fantastic.
Modern comics are starting to move away from this trend a little as better printing technology has allowed smooth gradients and shading to compensate for the issues of the past and opened up a whole new range of possibilities.
These are images from "The New 52" DC comics reboot.
"Look ma, no undies!" (To be honest, even here we see a fine line to make sure there is definition between pelvis and legs, but at least it doesn't look like underwear.)