That is partly because the treaty left out big emissions sources such as deforestation (see article), but also because potential participants were put off by the idea of internationally binding commitments.

ECONOMIST: How to get negotiations on the right track for a deal

Built into it was a stipulation that it would become legally binding only once it had been ratified by 55 separate countries, but also by enough Annex 1 countries to collectively account for 55 percent of global greenhouse gas emissions.

CNN: Kyoto: The next generation