Since each character can have punches in any of 12 rows, you can think of cards as using a (mostly-sparse) 12-bit encoding.
The sparseness of early punched-card formats has always seemed a little odd to me --- instead of storing up to 960 bits per card, this encoding only holds 480 bits (80 columns * 6 bits/63 characters each.) Perhaps people just didn't understand binary very well at the time?
The way that the mechanism works also reminds me of early teletypes --- they encoded and decoded characters using a similar system, essentially doing serial/parallel conversion entirely mechanically. Here's one example:
It's a function of the electromechanical technology: It's a lot easier and more reliable to have a wheel with ten index positions on it than to have four wheels with two index positions.
Many early electronic computers used binary coded decimal as a bridge between the electromechanical and the purely electronic worlds.
What does it feel like to type on?