It is simple economics, the US is the most powerful country in the world and it has a large educated middle class that influence how this country is run, 535 senators and congressmen plus the Obama administration run a country with a GDP of $15.7 trillion and a government that spends $3.8 trillion, so each elected official influences the spending of something in the order of $5 billion to $30 billion dollars.

Let's say 1% of this $5 billion is spent on educational grants and subsidies and you want to influence an official to spend 1% of this $50 million on your college fund for your disadvantaged ethnic group, that would be a cool $500000 in your pocket. Even if you spent $10000 on advertizing and rallying up support for your little cause you would still profit from this.

On a larger scale you might have nation wide special interest groups, these groups can determine whether someone gets elected or not, if they say you are unamerican/racist/sexist/homophobe, then "boom", you're out of the race. Both senators will try their best to promise to ensure the leaders of these movements get their kickbacks and funding.

Influencing people is big business and that is what the institution of the American left wing is all about.