There were several ways to answer the question. I took the biographical approach. I have already wrote about this when writing about how a Christian Fundamentalist like me became a Socialist too(read here) because I became both for roughly the same reasons.
But there are other factors besides biographical ones. Why I became an activist is the same reason why one of the Freedom Rider's felt compelled to protest the Jim Crow laws as they applied to travel in the South. When asked, this Freedom Rider said it was "everybody's responsibility" to work for the freedom to travel together, it is just that some are "more conscious" of their responsibilities than others.
So why did I become an activist? It is because everybody should. Everybody in America should being an activist is another way to exercise democracy. Everybody should because more and more people are suffering especially economically. Everybody should because our nation's resources are being wasted on one military intervention after another. And all of this does not include the serious environmental problems that we continually add.
Now the next question would be if everybody should be an activist, why are there so many Americans who are not? The answer to this question can vary and partially depends on one's religious views. The last blog post (read here) answered this question for many conservative Christians. But many more people are not conservative Christians so there is more than one reason why more people are not involved.
The next reason I can see has to do with the American Dream. As I understand it, the American dream is all about building one's own Fantasy Island. Of course, this dream island is built around escaping from rather than becoming involved with the world around them. And being distracted by this American Dream will continue for as long as people believe in the fantasy that we can live as if we are not our brother's keeper, that we really do not have to work for change until the economic bell tolls for us.
Here, we need to realize that because of our current political-economic system, we are experiencing a financial equivalent of what Pastor Martin Niemoller described under the Nazi rule of Germany. We could argue about whom the pink slips came for first but it doesn't matter. The dismissals came for the textile workers, for the air traffic controllers under Reagan, for those in manufacturing, for those with service jobs, for some with technical jobs , and so forth. Those who found themselves unemployed realized that their employers viewed them as disposable objects that could be easily replaced by slave labor or machines. Such people are now without a means to support themselves while the contentment of their employers and owners does not miss a beat. The main point is that while they do not come for us, we tend to look away and remain silent. Just as Niemoller said:
First they came for the socialists, and I did not speak out -- because I was not a socialist. Then they came for the trade unionists, and I did not speak out -- because I was not a trade unionist. Then they came for the Jews, and I did not speak out -- because I was not a Jew. Then they came for me -- and there was no one left to speak for meThough what Niemoller said is really a pragmatic appeal, our morals are on trial here regardless of one's religious views. Either we will speak and act in a way that proves beyond a shadow of a doubt that we believe in the intrinsic value of each person or our silence and passivity will be used against us to convict us of believing that people have value only when they meet our conditions.
Of course there are other reasons. Some have been hardened by the suffering they have been forced to experience. And there are those who currently enjoy wealth and power or who desire to ride in on the coattails of those who have it all.
Perhaps my expectations are unrealistic. That I live in my own fantasy world where people who are different from one another would actually care enough to speak out and sacrifice for each other. Perhaps what we need for my fantasy to become real is the kind of revolution Rachel Corrie wrote about when "The transformation that happens in an individual who goes from being selfish, insatiable, and ruthless into a person who is generally satisfied with the day-to-day and concerned with the well-being of the other people." 
We need both personal revolutions and public activism. Personal revolutions prepare people to be involved in the kind of activism necessary for humankind to change course. Those revolutions start with relationships and conversations and they culminate in publicly resisting wealth and power. They resist not just when their own group is threatened, but when any group is being oppressed and is forced to experience injustice regardless of the oppressor. The costs of these revolutions not taking place is currently so in our face that only those who deliberately shut their eyes and minds cannot see our ongoing collapse. The environment, our social structures, our use of brute force and authoritarianism, and our financial systems are failing and falling down around us. When do you think a good time to change is?
 Let Me Stand Alone: The Journals Of Rachel Corrie, pg 175