maybe (at least in the case of Cessna 100 series(and no not all!!!!)) it is placed on the left wing because
1 In testing this was the most reliable place to give an indication. (outboard of the slipstream and prior to the ailerons or outboard sections due to washout.)
It was probably also found that in varying a/c configurations and operating conditions (flap,slat,gear,speed,loading and bank) that the pressure envelope over that portion of the wing stayed adequately consistent with regards to its migration at varying angles of attack thus providing the most reliable information.
2 its a reed type warning and placed such that a pilot flying without a headset (as some used to and still do) has the best chance of hearing the warning.
vane types don't have this concern
3 the weight and plumbing of the warning, pitot tube and fuel vent are on the left to offset the weight of the flap motor on the right. (lets not chicken or egg that one)
4 cause that's where they effing stuck it...... You know how, why, where and when it works.........put some time and thought into preventing it being correct.
Its already been said though COST, WEIGHT, FUNCTIONALITY what more do you need to know for crying out loud.
PS number one is the most likely reason for the position of stall warnings