Same reason you don't see many left-handed infielders on the left side. Harder throw to make. Snap throws to first with a left-handed catcher wouldn't work very often.
That doesn't make sense, at least not for that reason. For infielders, a lefty would have to turn his body quite a bit to square up and make the throw to first, while a righty can basically just throw across his body. Looking at catchers, a lefty catcher would more easily make the throw to first because he can also throw across his body as opposed to a righty who must turn the body to square up to first.
Look at a baseball diamond and you'll see that a lefty catcher has the same basic throw to first as a righty infielder does. If anything, the throw to third is harder, and if you wanted to say that throw is more important I couldn't argue with you.
I always figured that the main reason is that a righty catcher has a more clear shot of throwing without the batter in the way the 2/3 of the time (or whatever it is) when a righty batter is up. So there's been that bias through the years, so there are very few or no catchers mitts for lefty throwers as kids are growing up, so lefty kids don't become catchers, and that bias perpetuates itself. Maybe that'll change with online shopping as I can find catchers mitts for lefty throwers online, but I can't ever remember seeing one at Kep Hardings or K Mart or anywhere else you'd find a glove when I was a kid.
And it's not "absolutely none". Benny Distefano caught in 3 games in 1989. A few others have caught a few games in major league history. No regulars.