A new research article from FNI addresses this question, and points to several possible answers. 

Researchers Kristin Rosendal and Steinar Andresen look at the relationship between the UN Panel on Climate Panel (IPCC) and its corresponding, but far less known 'sister organization', the panel on Biodiversity and Ecosystem Services (IPBES). The researchers ask the fundamental question: why has the Climate Panel gained so much more impact on politicians, the media and public opinion in general?

You can read Rosendal and Andresen's article on the subject here.