Just to hazard a guess, it's because you're using English or other left-to-right languages. Since we read left-to-right, we (typically) start lines of text on the left, so the left side is used more heavily.
Some printing styles will mollify this a bit -- printing right-justified text, printing mostly graphics, printing right-to-left languages, or printing newspaper-style evenly-distributed columns.
This is why we used to extend the life of our toner cartridges by pulling them out and shaking them, often several times, before completely replacing them.
14Would you happen to know of a study that calcutates the extended life gained by using the Justified alignment, which I could use to annoy my boss with? – mtone – 2010-01-13T21:53:30.937
3@monotone: no, but the concept is hilarious. – quack quixote – 2010-01-13T21:55:43.177
3@monotone - someone should write a thesis on that !! :) – Rook – 2010-01-13T22:43:38.277