I don't think it's that complicated: because not everyone thinks "being happy" is the utmost peak of meaning or purpose. Many people desire challenges, difficulty, a grand narrative that comes with ups and downs.
And those people exist and are handled specifically in Brave New World. It's just that the system fucked up a bit and took a long time to find the protagonist of the book. But consider the world you and I live in where a vast majority of people who want challenges like this can never have them as they are stuck being dirt poor, or in criminality or something. It's gotten a lot better in the last 100 years, but it's no where close to BNW.