When the first comic-book (or comic-strip) superheroes were created, in order to give the reader the idea that they were stronger than normal people, the costumes were patterned after the circus-strongman attire of the day: a body leotard, with trunks, shorts or briefs over it for modesty purposes.
Traditionalist though I am in a lot of things concerning comic-book heroes, I'm actually glad that they're moving away from the "underwear on the outside" look.
I'm not. There are still plenty of wrestlers who wear that. It doesn't look "out-dated" at all. It looks like a practical solution for somebody who wears spandex leggings while involved in acrobatic action who doesn't want their pants to fall down.
Superman's outfit without them looks idiotic.