Why AC rated in Tons, Not in kW or kVA?

AC (Air-conditions and Refrigeration are always rated in Tons.
Air conditioners are always rated in Tons capacityinstead of kW because Air conditioners are designed on the basis of quantity of heat removal from room, hall or specific area. Quantity of heat is termed in Tons means if an air conditioner is able to remove 1000 kilocalories of heat or 4120 kilojoules or 12000 BTU of heat in an hour that AC rated as 1 Ton of AC because 1000 Kilocalories or 4120 kilojoules or 12000 BTU equal to one Ton of heat. Also, this is the same case for freezer and refrigerator i.e. refrigeration system.
Good to know:
BTU = British thermal unit. A measurement of heat, specifically, the amount of heat needed to raise the temperature of a pound of water by 1°F.

AC (Air-conditions and Refrigeration) are rated in tons coz if u start rating them in kw there is no direct way u can specify its capacity,as 

for example 

1 ton = 12000 BTU 
1kw = 3412 BTU 

so it means 1 ton ac have =12000/3.412 = 3.516 kw

Air conditioners are rated in KW.
Both terminologies are simultaneous. Compressors are rated by HP and KW.
‘TON’ is to show equivalent heat transfer capability of the system which takes refrigerant’s properties into calculation, and definitely refrigerant is not rated by KW.
So, both ratings are important.
KW or HP from electrical point of view and ‘TON’ from heat trasfer perspective.