She was actually going to be promoted, but she declined promotion, and wanted the "Honorary Lifetime Kenkyuusei" badge instead actually (only girl on SKE member page with this title: "SKE48 終身名誉研究生").
Kaotan does for SKE kenkyuusei what Sasshii does for HKT48: She's a leader, a mentor, a senior, and she is outrageously extrovert, not afraid of any situation really, and through these skills, she pulls with her the other new girls. You might say that well, this could hamper her career, that she cares too much about others instead of herself - but luckily it doesn't because she has become a cult-figure through the way she is, and that she's also a kenkyuusei still. And while she promotes herself, she also promotes the other girls, so it's a win-win situation for management.
So it's like what Erin. says, she's a leader and an influence. I think if she got promoted, she would lose a lot of that "Still a kenkyuusei..." hype she had. She'd potentially be "just another senior" member, with the challenge of how do you stick out from the rest now?
She found her niche, and ran with it