More people pay attention to celebrities than to politicians. Although politicians have a greater role in shaping public policy, celebrities have a greater sway over the public and a larger influence about how people think and what they do with their time and money, thus making them more effective when it comes to social justice.
Politicians do not have a good effect on peoples' willingness to be more socially aware. In fact, they actually have a detrimental effect because the public is generally not inclined to believe what politicians say. In contrast, people are attracted to celebrities and view them as role models. This encourages people to act in a socially responsible way if they see someone famous that they admire behaving similarly.
Nothing much can be said about about politicians, as a far as am concerned they will be vocal on issues that will one way or the other affect their career. Celebrities on the other hand enjoy massive following so when it comes to raising their voices against injustices they mobilize people in numbers and this has nothing to do with them, it is for the interest of the people.
Even though celebrities can be influential in many areas, even politics and social justice awareness, they are not generally considered experts in that field. It is easy for Americans to dismiss celebrity opinion as somewhat meaningless and ill informed. Politicians, in contrast, are considered to be experts in their field or at least well informed in regards to public policy's effect on social justice. Politicians are also perceived to have the ability to affect real change in regards to social justice thus people are more interested in hearing their opinions.