I am a southpaw and started using a mouse a very long time ago, so frankly I do not recall the reason exactly. My initial use was always with a QWERTY keyboard and my dexterity on the keyboard one-handed was much better with my left. read more
And on top of that, Logitech’s research indicates that a good number of left-handers actually choose to use a right-handed mouse. In other words, the actual market for left-handed mice is quite small. (As a side note, I was curious why a left-hander would choose to use a right-hand mouse. read more