I'm not up to date on what's been going on with the show, but last I saw, it's also the one that lends itself to the most "how the heck does this happen" fanfic premises. Like, if Ladybug expresses interest in Adrien or vice versa, making the other one not go for it requires contrivance. If they're both supers or both civilians, there's fewer identity shenanigans, and ML is all about identity shenanigans. But to make Marichat work, you need to start with explaining how you get there, and so it sparks interest.