Why is it difficult to identify any single cause of climate change?

It is not difficult. The one single cause of climate change is global warming.

There are several causes of global warming, but the two main causes are the burning of fossil fuels (coal, oil and natural gas) and deforestation (cutting down trees which used to remove carbon dioxide (CO2) from the atmosphere). Both these mean more greenhouse gases in the atmosphere, and this is what is causing global warming which is causing climate change. Not difficult to identify at all!