Disclaimer: I am not a comic book/superhero guy, but my toddler learns about them from school, friends' toys, etc., and he loves to put on a cape and run around the house shouting "SUPERHERO TO THE RESCUE."
So, why are superheroes depicted with capes? (I know, not all superheroes, etc.) Was there something about the artistic milieu of the 1930s, when Superman was created, that led to this, and are later heroes knockoffs of Superman? Or is something different at work?
Superman was definitely the trendsetter here, and others followed. Note that the next big characters with capes were Batman, coming from the same company, and Captain Marvel, who was such a similar character to Superman, DC successfully sued. Meanwhile, characters in Timely/Marvel comics were successful without capes (Captain America, Human Torch, Sub-Mariner), but that did not stop Timely from introducing many characters with: The Angel, Blue Marvel, Blue Blade, Thunderer, Black Widow, Phantom Reporter, to name a few.
As for why Superman had a cape?
/u/Comics_and_Pulps had this answer, and I agree, Seigel and Shuster were influenced by Flash Gordon:
As an additional question: I've always heard California Batman Zorro was a very large influence on the superhero comic book genre. Is that reasonable? Also, he isn't mentioned in /u/Nejfelt's linked anwer, but was also explicitly caped, would he have been an influence on the prominence of capes?