I also thought the image of sumo is that of a tough manly man sport and having sexy women do it would feel weird. Wouldn't it be like if we had scantily clad women do american football/rugby? Would anyone find that sexy or weird?
I figured that's why few Japanese BBW works tap into Female sumo, they view the sport as a manly man thing so putting women into it and sexualizing it would feel more weird than sexy to most creators and to a potential audience. There is a cultural/religious thing to it too so putting sexy babes into it would likely be seen as disrespectful as well as being too weird to find sexy. The sport has religious ties so sexualizing the participants and stuff related to it could be seen as a way to anger a lot of people.
I also see that not a lot of creators are interested in the sport, so doing loads of works/characters related to it wouldn't interest them, like not all americans are into american football/baseball. I note that most sumo characters aren't portrayed as sexy/attractive, male creators can certainly do beefcake, guess that for most straight male mangaka, sumo wrestlers don't fit their idea of beefcake.
Plus japanese women are expected to be delicate, lithe and demure, the exact opposite of a sumo wrestler. So most Japanese men wouldn't find the idea of a female sumo wrestler to be sexy. I mean, yeah I know there are creators that are into muscle babes and female wrestlers/action babes, I guess the idea that sport is dude only makes the idea of female sumos something that most mangaka aren't willing to do. It disregards basic rules of the sport, defies religious traditions and sexualizes something that is meant to be serious. Cause sexualizing female wrestling/having action babes is seen as more appealing and doable over having sexy female sumos.
Maybe things can change, if Plus-size elf could get made and have an anime made, maybe one day, we'll get a story of sexy female sumos, and to depict them gaining weight as sexy. I like to hold out hope, it's annoying to have a million people be angry and negative about this. If Plus-sized elf could happen, maybe this story could happen one day too.