This is me just spitballing and oversimplifying, but it would seem to me that he still had some good athleticism and baseball instincts, which would serve him well in the outfield. But just a slight decline in reflexes, bat speed, etc. can tank someone at the plate. There's more room for error (no pun intended) in the outfield than there is at bat.
EDIT: Not exactly a comparable example, but as long as my legs function, I can still run after and catch a fly ball (obviously not like a pro player can, we're talking in relative terms), but there's no way I could hit a pitch even in the 70-MPH range anymore without completely selling out and swinging before I even see the ball.