IMHO, for the same reason many of them are interested in Buddhism. I think many middle-upper class people struggle to understand why they are not happy in spite

IMHO, for the same reason many of them are interested in Buddhism. I think many middle-upper class people struggle to understand why they are not happy in spite of their material wealth. Buddhism, Stoicism, and other philosophies offer answers for how one can find happiness/satisfaction in life, irrespective of one's material possessions or status. I also think that Stoicism in particular may currently be popular because, in comparison to eastern philosophies, it is more grounded in traditional Western thought and may be easier for Westerners to understand. At least compared to my efforts to understand Taoism, for example.