Of course, the fact that Olaudah Equiano was a former slave who had taken it the intiative to educate himself and free himself from the chains of physical and mental slavery made his message more effective and powerful. Equiano, however, had other characteristics that appealed to the British and eventually to American readers. He moved to Britian after earning his freedom in part because Britian was much closer to absolving slavery at that time, and he knew that he would be able to make more of a difference there. He married a white woman, not for political gain, but in his world, this did help him gain an "in" with some who might not have helped him without her. She tirelessly supported him and his cause. Finally, Equiano was savvy in who he worked with and did not cave to the pressure put on him by powerful members of Parliament and British society.
One last point--Equiano had experienced firsthand and wrote about traveling on the slave ships. As a boy of privilege in Africa, the conditions of his voyage would have been foreign and even more horrific. He found himself in a world that he could not have imagined. Equiano took that experience and described it as often as possible to stress how inhumane the process of just getting slaves from one point to another was. He moved many people to get involved because he could provide primary knowledge and experience that many other abolitionists could not.