A variety of reasons, but I think generally he just wanted to have a "normal life." I don't know how comfortable he was being treated like a commodity, and during the height of Journey's popularity in the early to mid 80s their schedules were relentless. This took a toll on their personal lives and relationships.
So the Journey was definitely a tough one. Without his promise to his dying mother I don't even know if we would have gotten "Raised on Radio." I had some hope for "Trial By Fire" and their reunion but again he was treated like a commodity when the rest of the band wanted him to have major surgery so they could hit the road.
He honestly felt that wasn't a band decision, but a personal one.
So after that I think he was content to fade back into the background. He moved to the beach and generally keeps a low profile.
You can catch up if you want on his Q&As that he posts periodically on Fan Asylum's page:
http://fanasylum.com/steveperry/
This last birthday he did a really in-depth one so check it out. Nice to see him peek from behind the curtain every now and then.
"Whether you think you can or you think you can't, you're right." - Henry Ford