Health. Hyperthyroidism is bad for heart/anxiety. He also has a family and when you've been in martial arts/movies as long as he's been, side roles make enough bank to keep the lights on. Tbh, he doesn't need a reason other than because. He's been in side roles with Mulan and Expendables, so he keeps busy. He's just not headlining anymore. Meh. Martial arts takes a huge toll on one's body. I mean, look at Jackie Chan. Even when you do it right and prep, things go wrong. He's broken quite a few bones. It adds up when you get older and he's almost 60.