The only exceptions seem to be IU and Eunji, everyone else either completely abandons or drastically reduce their work in music.
Workload, profits can only be part of the reason.
If they genuinely enjoyed perfoming music, why would they drop it so quickly?
Not even Youtube covers etc.
Edit:
Seems most responses agree on,
#1. They weren't into music in the first place, but just a celebrity life.
#2. Being an idol doing constant fan service, subject to the policing done by fans about their morals and life choices, is not enjoyed by most idols. It is just an act they put on and with a better option in actual acting, they are quick to dump idoling.
#3. Being an idol is more hard work and restrictions- diet, practice, relationships, so they escape it as soon as.
Valid points.