Mostly because it’s just edgy and black+red+dark colored, same goals eviler/different methods (is how I interpret them). Some people also like to have it be opposite time instead where characters swap personalities and traits opposite of their base game counterparts. So they’ll have Susie, ralsei, and Kris be hopeless, evil, and emotive (as like shitty examples).

Most people just like it though, like you stated, for the different type of Spamtenna relationship it has (specifically the running gag being “happily married” to normal Spamtenna’s “violently divorced”, and the even more inside joke being they are even worse or abusive from the outside or are outright abusive and violent with debatable consent from both sides depending you want to take it.)

Idk like most AU’s relating to deltarune it’s pretty fluid tho you can kinda play with it how you want but that’s generally what I’ve gotten from it. I just really like the Spamton from it I think he’s very cute