I guess it just feels like some of the authors marking their novels as slice of life are picking the genre because the story didn't fit anywhere else or they didn't want to have it compete with others in a genre that fits better. And the genre is hard to define compared to others so who's to say it isn't?
I mean, Mystery- you expect a mystery to be involved, Romance- there's a romantic relationship, BL- that relationship is between guys, Horror- it's scary, Fantasy- there's probably some sort of magic
Slice of life? It's a slice of someone's life, whatever that means....
Like, is the premise "I woke up in another world to find myself married to multiple guys" really slice of life or did they just not want to be buried in all the other fantasy or romance stories? Or there wasn't a better genre for a story revolving around time travel? What about the rollercoaster of "character is worthless nobody, family comes forward to reveal the whole family are super important, oh but years later character finds out they're not actually related, but the rest of the family doesn't believe being blood related defines a family"....?
It's like the category is being used for just whatever the author wants to loosely define as "slice of life" and I wouldn't be surprised if that plays a part in why the genre doesn't have enough popularity to have its own section on premium novels.