Few popular reasons, widely supported by the critics, for the failure of the Kyoto protocol are as follows.
The negotiations for the protocol began in 1992. It was adopted in 1997. There was already a 5-year time lapse in the life of the treaty. Moreover, the ratification of the protocol by the different members happened from 1998-2010 (a 12-year period). This left very little time for the protocol, especially with its ambitious targets.
Even though the US signed the protocol, it refused to ratify it citing economic and scientific reasons. Being a developed, industrialized nation, its refusal to participate was a big disadvantage for the treaty. Kyoto protocol, in a way, failed to persuade the superpower and the largest emitter of greenhouse gases.
The developing nations were exempt from stringent targets with the aim of not hindering their growth. However, some nations like China and India are large emitters of greenhouse gases. Not restricting them actually hindered the protocol from reaching its target.