It's not always just them, the problem is like most fanfic the Reverse AU fanfic started focusing on mostly shipping. There are exceptions now but this always becomes a big trend. In most AUs or regular stuff the big Gravity Falls ships are Padippica, Billdip, Mabifica, Pinecest, sometimes Mabill. Even stuff with the grunkles seems less common in the big AUs than usual for some reason and there's surprisingly little Wendip fanfic in general. So Padippica fanfic is just Reverse Pacifica and Reverse Dipper, Reverse Pinecest is just Reverse Dipper and Mabel, Japan's stuff will be mostly Reverse Mabel and Reverse Bill. Fine for shippers but bad for worldbuilding. I know that some non-shipping fics do exist though they just seem less popular. So those four characters, show up a lot, even though the idea started out with Mabel has a crush on Gideon in this version, hardly anybody ships any versions of Mabel and Gideon, so you don't see him.
Now the generic cliche one off Reverse Falls illustration, drawn by artists who just make one picture for the AU and that's it, and are often really good art, is just Reverse Mabel and Reverse Dipper with Reverse Bill, sometimes magic glowing blue stuff and Bill on glowing blue chains. Why, the answer is simple. They are all blue, it's just about the aesthetic for an illustration, the outfits twins wears looks cool and yes the Mabel has a sexy outfit. It has boy character girl character and mascot character. That's all some artists even know about it.
So this image became a literal visual meme. It spread across language barriers to other countries. So you often see this meme image from other countries, it shows up on pixiv a lot. A few people will look into it and see what else is to it but not all, the fourth character an artist like that might draw will be Reverse Pacifica. Why? Probably shipping, she is shipped with both the twins all over the world (like Bill is), Pacifica is cute and fun to draw.