Like many customary units, the ton (of cooling) is tied to historical accidents and I dare say is easier to imagine for a layperson; one could visualize the coolness rushing off a cubic-meter block of ice in a room, and how quickly it would melt depending on the temperature. 12000 BTUs, in the other hand, is not as intuitive (and the BTU itself is a customary unit, which is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit).

And further, I guess there's just not much to be gained by going metric and saying "14 kilowatt AC unit" vs. "4-ton AC unit".

Since electricity is metered in kilowatt hours, it's much easier to see the running costs of a device with a power given in that unit.

When picking an AC unit, running cost is not the primary concern: sizing is the primary concern. If you pick too large or small of a unit you will have issues. Too small won't be able to cool, too large generally won't be able to bring humidity down (outside of the newer inverter units that can run slower for longer).

Tons (or, BTUs of cooling) makes more sense, because the efficiency for a given cooling amount is variable.

For efficiency, we have efficiency ratings in the SEER/SEER2 ratings.

Kilowatts are ambiguous between amount of cooling and amount of electricity used.

Rods to the hogshead and I like it!