I believe two reasons: first, political will. Fossil fuel companies are large and entrenched, and have lots of experience lobbying governments. They block things like carbon taxes.

Second, a strange sort of game theory where each player (each country) thinks “My individual contributions to greenhouse gasses are just a small part of the total. They won’t cause global catastrophe. Just an incremental increase in the existing catastrophe. The incremental harm won’t fall directly on me; it will be divided among many countries. If continuing to use fossil fuels provides some small economic advantage, it outweighs the portion of the harms that will land on me. As for the harms I experience from other countries’ carbon emissions, there’s nothing I can do to prevent them.”