The reason for this has to do with what we can or should do about global warming. The more that global warming is natural, the less it is necessary to change our ways to curtail it. The more that it is anthropogenic (caused by people) the more we are obligated to prevent it from continuing.
In one sense, we might say that the causes of what we now call climate change do not matter. If climate change is going to have bad effects, it does not matter whether those effects are caused by people or by natural processes. One way or another, we need to prevent those effects. We need to do something, for example, so that low-lying areas do not get flooded by rising sea levels.
But the causes of climate change do matter. First of all, if climate change is anthropogenic, then it is clear that there is something that we can do about it. If we are causing changes through excessive use of fossil fuels, we must reduce our use of those fuels and thereby reduce the climate change.
By contrast, if changes are caused by natural processes, there may be nothing that we can do about it. Second, if we are the cause of changes, we have more of an ethical obligation to stop it. If low-lying coastal areas (or entire island nations) are inundated by natural causes, it is just too bad. But if they are being flooded because of our actions, we have an obligation to act to do something.
Thus, the causes of climate change do not alter the impacts of that change. However, they do alter what we must do about that climate change.