Because they're too pleb-like to be Advanced, and too good for Free, whereas Advanced people are just good, and Free people are just plebs.
Lol, but probably simply because it appeals to the largest audience, like most things that are not too serious, but also not simply ridiculous. Plus the term "casual" is simply appealing from a psychological perspective. The term "casual" is a naturally inviting word, often embodying a calm or carefree sort of style.
Furthermore, most people don't RP for the sake of just making a story or having fun, they like to do a little bit of both. I feel like Advanced and Free sacrifice one of each of those elements. Advanced is, in many cases, more about the story than anything, and it's sort of restrictive in that sense. That's not to say it isn't fun, but it does put more dampers on what people can do than most casual RPs do. On the other end of the spectrum, Free is, well, free, and also often chaotic and without a good baseline. Literally anything can happen, with very little regard for retaining any coherent storyline.
Naturally, Casual is a good middle ground, and it is really more broad than just comprising 33.3% of the RP community on here. Casual holds a broad variety of RPs, some of which could even be mistaken for their Free or Advanced counterparts. Really, it all just depends. The Casual section is very versatile in that way, making it appealing to many people for that reason. It can be manipulated fairly easily whilst still maintaining a sense of dignity.
Also, as someone who almost exclusively roams the Advanced section, I will say that there is possibly an element of elitism at play, but I also honestly, legitimately think that Advanced and Casual are both devolving somewhat. I've been in both Casual and Advanced RPs that I had high hopes for, but ended up disappointing me a lot with their too casual nature. I like having something concrete to work with, and while open-world shit is cool too, I feel like it needs to be moderated so as to avoid it being populated with, like, 80% shitty, underdeveloped ideas that end up going nowhere and having no bearing in the actual story.