There are four possible explanations as to why there are (virtually) no left-handed catchers:
1. The counter-clockwise flow of the game. Runners from third are coming in on the side the left-hander is catching the ball,. As a result, the left-handed catchers could not effectively block the plate and simunltaneously protect the ball. They also might be dangerously exposed to the runner, with the the left leg especially vulnerable to injury.
2. Obstructions throwing to bases. The argument is that throws to third are obstructed, although right-handed catchers would have the same problem with first.
3. No left-handed catcher's mitts.
4. Mindless devotion to baseball tradition.
Here's an article on the subject:
Left Handed Catchers [ April 08, 2003, 07:43 AM: Message edited by: Charlie in the Trees ]