Maybe he got bullied or smthng. Like yknow, he's dojin's son and so on
My theory
It was mention that he dropped it after he found out he was an alpha and how others also thought he was gonna be an omega. We know how the idol industry is when it comes to bands so my guess is they had this image of what byul would be to debut as an idol and when he's sex didn't line up w that (considering hes more on the cute side and being an alpha would "take that away") he realized he wouldn't fit with that image so he quit before being told straight up that they didn't want him
I think because he grew after becoming an alpha and he's no longer small and dainty and cute size.
I wonder why Byul gave up being an idol when both his parents will surely be supportive of him and that is what he is good at.
Maybe he don't want to be called nepo baby ╮(￣▽￣)╭