A lot of it is just common usage.  I love, love, love Golden Age Jungle Girl comics, but I rarely hear  them referred to as "superhero" comics.   
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Ok, I can't leave well enough alone.

(A)  Linguistically speaking Jungle Girl-type comics are not superheroes because they were fairly popular in the early '50s, and the language we use to discuss superhero comics ("Golden Age" "Silver Age") discounts the idea of "new" superhero characters in the in-between-gold-and-silver age.   (Yeah, I can think of half-a-dozen counter examples too.)  

 (B)  Also there's pure sexism.  Sheena and Rina type characters are, well, girls.  And superheroes are historically masculine power fantasies, and fanboys are not going to want to admit that, for a couple years there, a substantial percentage of "superhero" comics starred women.  

(C) Aaaand it does feel like the jungle girl books were marketed to a different audience than superhero books. If you look at genre-categorized DC subscription ads from the '70s, Rima is going to be listed under "Adventure" or "Jungle" not "Superhero."  (Except maybe in the 6 month period where she showed up on SuperFriends.)  Listen, I'm with you, on some level all "genre" labels are artificial marketing constructs that are anti-art.  But virtually everybody uses them so it's generally not worth the effort to fight.   Jungle Girl books are not called "superheroes" in in-house marketing materials.  

(D) Jungle Girls predate Superman. You can argue that Supey was not the "first" superhero but he was 100% the trope codifier for "superhero" as a pop-culture concept.  The only pre-Superman characters who are inarguably "superheroes" are highly superhero-adjacent, like the Crimson Avenger.   (IOW, CA regularly hung out with Superman derived superheroes.  And changed to be less like the Green Hornet and more like Batman as his career progressed.)  

Although I think I'd argue that the Green Hornet was a superhero.  *Shrugguy*  That might torpedo my argument.  

(E)  By and large superheroes don't spend the majority of their time in exotic locations.   At the very least they maintain a home-base somewhere in the US or England or somewhere that would be called "civilized" in 1942.  (The lines blur a little bit.  There were times when the Black Panther was as much a Tarzan style Jungle hero as a superhero.  But not his first appearance, which is Afrofuturism in it's purest form.  The Tarzan-y got added in his second major appearance in Tales of Suspense.)  

(F)  Sheena didn't have a secret identity.  Superhero fiction is based on audience identification, and Jungle Girls weren't really about that.  (Also see point E above.)  

ANYWAY, my point is:  (I) Because of these factors Leopard Girl was not reintroduced during the sixties Marvel superhero boom.  (II) This is the greatest tragedy in human history.  

Regarding that point, the earliest example I've seen of a character that I'd agree is a superhero is Spring-heeled Jack, as he seems to use an alter ego, costume, and extraordinary abilities for the purposes of fighting crime:

Yeah, she's the same character, though I'd rather use "Gimmick Girl" to help keep the list as simplified as possible (as in shortest distinct names and no extra commas).

Spring-Heeled Jack wasn't a superhero...he was a figure of terror, like a vampire, preying on pedestrians. He was an urban legend (which may have had some basis in actuals predators, that got exaggerated) that got turned into Victorian Penny Dreadfuls.

If you want to go into the Victorian era, for superhero ancestors, you can look at Sherlock Holmes, Allen Quatermain, Fantomas, the Nyctalope, Arsene Lupin, Raffles, The Scarlet Pimpernel, Zorro, Tarzan, John Carter, Sar Dubonotal, Dr Syn, Dr Jekyll and Mr Hyde, the Invisible Man, Fantomas, etc. They borrowed from myth and folklore and spawned the pulp magazines of the 20th Century, which begat the comic book adventurers of the 30s and 40s and the superheroes who followed.

The French silent movie serial anti-hero irma Vep, from the film Les Vampyres, is one of the real templates for the superheroine. She is a super-criminal, dressed in a black bodystocking, prowling the roofs and mansions of Paris, stealing for the criminal gang, The Vampires (her name is an anagram). She battles a rival in the gang for supremacy....

(Here she runs into her rival...)






Her image likely inspired this famous Weird Tales cover....

Some fiction published later in the 1800s and early 1900s portrayed Spring-heeled Jack as a hero rather than a villain, but I suppose I'll just leave it at that.

And regarding pre-Golden Age pulp heroines, there's also Domino Lady from Saucy Romantic Adventures.

What, no Ocelot Lady?  Someone just didn't appreciate other jungle cats....

Entirely possible that someone used him as a hero; the only references and examples I have seen, from the Victorian era, had him as a villain, ala Sweeney Todd.

Re:  Domino Lady-the only really prominent costumed mystery woman of the pulps.