Because, ultimately, that's what a lot of oppression comes down to-- when people are marginalized financially. It makes sense, on a basic level. First, have the ability to hold a job and feed a family. Second, voting rights and the like.
Spurred by an anti-consumerist magazine’s call to action (and operating under the slogan of "We are the 99%), for the past three weeks around 5,000 people have marched on America's most famous symbol of the higher-ups in finance.

But why did they start this? What do they want?
The first question is not terribly difficult to answer. Most of the United States is not in its happy place, economically. People are not very confident in the economy's future, and unemployment is at an undesirable 8.7%. It is certainly not unfathomable that a large amount of people would take to the streets in protest. Again, back to the nucleus of a lot of oppression-- the ability to provide for oneself and one's dependents. Doesn't seem unreasonable.
This leaves the second question: Why? What do they want?
The protest movement has been criticized as unfocused. It is modeled after the Arab Spring, yet the recent Middle Eastern protests seem to have the pretty clear goal of government overthrow/reform. It would be easy to dismiss 5,000 people in the largest city in the country as ignorant, disenchanted youth.
But anyone who calls these protests unfocused is clearly not listening. Occupywallst.org, the "unofficial de facto" voice of the protesters, released both a list of grievances (cleverly modeled after the Declaration of Independence) and a popular but unofficial Proposed List of Demands. The grievances are very broad but include workers' rights, education, and how money is handled; the demands, while equally far-reaching, are mostly for legislation for responsible banking and to minimize the influence of corporations of lobbyists on elections and the democratic process.
Of course, the protests are not as simple as that. There has been some debate over the role of unions in the protests, and many Republican politicians have expressed their contempt for the movement. But is there anything really all that reprehensible in the Occupy Wall Street sentiments? Don't most people agree with the basic tenet that corporate greed is bad? Or are they asking for too much?
I'd love some feedback concerning your personal feelings regarding the protests. Feel free to leave a comment. =)