Newb question.

So I'm about to buy a frame & goodies to start on my first build. Eventually I'd love to get an old school Campy Pista crankset. I've seen several out there with prices ranging according to condition. My question is, why the hell are there tons of right crankarms but no left arms? I know there's gotta be a logical reason. Thanks in advance.

PS Be warned, I'll probably have more dumb questions as I start tinkering about.