Because the sun is nowhere near as well understood as people think it is, and it is the main source of heat for the Earth.

The sun does change its behavior, such as the Maunder minimum. How this affects the Earth is not well known, if for no other reason that the sun has been in the same mode since modern instrumentation started observing it.

To simply dismiss the Sun's role in climate change is foolish. It is about the only thing that if it does change, there will be an effect.