I believe the original reason was that it took more die drawings to produce smaller wire and the number of draws were counted and attributed to the specific wire size. For example, maybe 10 gauge wire took 10 individual drawings through incrementally smaller dies to make it that size. 20 gauge wire would take an additional 10 drawings to manufacture, and so on. Those "measurements" are long gone and have no bearing on today’s wire sizes other than to have started the general trend which is to name smaller wire with a larger number.
As far as current wire gauge standards- one of my favorite things.....standards. Standards are good but what is great is when you have more than one, and preferably something like wire gauge sizes where there are many standards! Brown and Sharpe (just down the road from me) standardized wire sizes.... first. Then all the others came along and use names like AWG (American wire gauges) which should never be confused with the Imperial measurement (British) called Standard Wire Gauges. There are a pile of 'standards' and so wire sizes are always specified using the 'standard' used (30 gauge AWG for example), and that is usually followed by an inch or millimeter designation.... just to be sure.
We should probably talk about machine tapers next.... Morse, B&S, Cincinnati, NM (National Machine), Hardinge, etc., etc. ad nauseum. None of them interchange and most of them are not even close to another is size / shape except for the ones that are but they still are not interchangeable. It results in a lot of "You cannot stick THAT in THERE." on shop floors. If you hear that anywhere else it means something else.
The world of measurements and quantifications is a mess I tells ya' and the only thing left to do with it is to get a chuckle here and there.
Brian
That leads to another question Brian.
Why are wire gauge sizes treated the same way as shotgun gauge sizes?