There’s a nice little article here about why Superman wears his underpants on the outside, which comes to the answer that the crude printing techniques meant that designs for characters had to be segmented and multicoloured, or it would be hard to understand what was going on in the picture.
But what about above the waistline? Why’s Superman wearing a leotard? There are a number of explanations. The reason that I’ve heard a number of comics’ scholars give is that it’s based on a circus strongman outfit. I’m sure strongman outfits came in all sorts of colours and designs, and I’m sure some may have had capes … although I’ve never found one.
Here’s Superman fighting someone in a strongman outfit:
I think there might be another solution. I was listening to the Knights of the White Carnation, a 1947 Superman radio serial. In that, we’re told Superman changes into his ‘jersey’.
Here’s a football jersey from the 1940s:
… and this is what Superman looks to be wearing in the early live action appearances:
Now, in the comics and animation, there tends to be a cleaner line, and the early versions are fairly light on detail generally, but it’s a few years down the line before Superman seems to be wearing something skintight that shows off all his muscles. Early on, he seems to be wearing something a little thicker. He fills it out, sure, but you don’t see every contour of his body.
If true, this still doesn’t explain the boots or the tights, of course …