There are several reasons why. One is that it's a matter of convention, and a variety of entities, countries, scientific panels have used it. I probably am not totally aware of all the reasons they did that, but I do know one of the reasons they did it. It's based on the principle that it partly has to do with the timing of the original Kyoto proposals. That is one.
The other one is there are countries and there are businesses that believed early on they should move in that direction. There's a strong feeling in certain circles that they should not be punished for having moved more quickly.