Because The Cheetah is her Main villain. This is like asking why do we keep seeing Lex Luthor. Barbara Ann or even Priscilla should be treated as her Lex Luthor/Joker. And the more often we see her, and she's treated with respect, power and fear, the more she can turn into that type of villain for Diana.
Richard Alexander
Yep, and since Patty Jenkins seems set to focus on the personal relation between Barbara Ann Minerve and Diana, and not only on the Wonder Woman–Cheetah antagonism, it's really well suited to a two-film arc where Cheetah is the emerging villain/monster in the first film.
In a way, current-day Diana and Cheetah are much like Professor X and Magneto: powerful people who violently oppose each other but still remain friends in more than one way and deeply respect each other.
Villain churning has become part of the MCU formula, but it's not the only way to make good superhero films.
«Speaking generally, it is because of the desire of the tragic poets for the marvellous that so varied and inconsistent an account of Medea has been given out» (Diodorus Siculus, The Library of History [4.56.1])
I can only see the Cheetah as someone that got knocked out by Batman by one punch (dismissively) and outfought by a non superpowered Catwoman.
Not really the worthy arch-nemesis that Wonder Woman should have.
But hey, maybe Diana will fly in the movie, albeit not under her own power.
If that's all you can see is two maybe three instances where Barbara Ann was written horribly, instead of the many other issues,especially as written by Perez, Simone, Jimenez, Johns and Rucka, where she was written as a terrifying powerhouse, well that's pretty bad and you are only seeing what you want to see.
There are far more stories with her written correctly than incorrectly.
Richard Alexander
No two arch enemy match ups are the same. When people compare them to Xavier/Magneto it's because that's another classic hero/villain match up with a complicated love/hate dynamic. Not simplifying it to her literally being Magneto with tits.
She's nowhere close to a Dark Phoenix. And that's not a problem at all, as few other rogues galleries even have a figure like that.
The problem is you're reading appearances by her by Batman writers who don't give a crap about Wonder Woman and letting them dictate how you view the character. Read Perez, Jimenez and Rucka. And Batman will not be appearing in this film, and the majority of people who will see it will never pick up a comic book, so who cares?
Or blame Marston. He's the one who established Cheetah as one of her primary enemies, which lead to subsequent usage and revamps. It's not a matter of "should," she straight up is the arch nemesis thanks to comic book survival of the fittest. She made it from 1944 to now when few other recurring enemies did. And the other contenders for the throne, Ares and Circe, aren't exactly swimming in great stories themselves and the latter has nowhere near the other media presence.