The usage of the ton measurement of cooling went out many years ago, not heard that used probably in 20 years. This is mainly because it is rated base on the cooling capacity. no matter how efficient you build it. In addition to this, Btu/hr, tons, and kW are all different units of heat transfer. Just like degrees C and F are units of temperature, and metres and feet are units of length.
Finally, the main reason is that it is really a long story. However, I will give simple explanation. For example, we rated AC in tonnes because how much ice is required to cool 1deg surrounding temperature. That is why we rated it in tonnes.
