He wasn't too good of a pro and probably figured out he'd do a better job managing his more talented brother Jake.
Because when Joey fought Anton Raadik who was a pretty good but not great fighter, and who might have been a step up in competition over Joey's previous opponents, he lost in fairly one sided fashion. Jake went on to face Raadik in order to avenge Joey and beat Raadik pretty easily. Joey then lost to Flores twice in a row and I think at that point he realized that he wasnt the boxer his brother was and would probably never graduate much beyond being a club fighter having to settle for undercard bouts on Jakes cards and gave up the sport. A smart decision on his part. One of the few fighters who quits at the right time.