I think that's because most players are veteran to RO, they proved they could grind their way to 99/70, and since private servers sometimes shut down they did it multiple times usually. So the same old grinding is not appealing to them, and they turn to midrates.
Maybe also because the RO population has aged a little, from teenagers in high school to young adults in college who have less time to play video game and are seeking a more casual experience.