Many people who work for the federal government of the United States earn little to nothing (interns) as far as a salary. For example, for my Congressman has staff members who make less than US$20,000 per year. I don't know how easy it would be to live in Washington, D.C. on an income that is that low.
For someone to be a summer intern, for example, that person needs to have the financial support of his or her family. Many working people do not earn enough money to support a child spending the summer in Washington, D.C., even if it is for educational credits. The same would be true for students working their way through college.
So, the people who are able to intern for members of the Senate and the House have to come from relatively well-off families. The same is most likely true for those people working as legislative aides for relatively low pay.
It seems that this prevents people from lower economic levels from getting into the staff positions in the federal government.
Also, with the people who are helping members of the Senate and House determine policies making so little money, it seems, at least to me, these people might be more easily to lobbyist efforts. A lobbyist taking a legislative aide to lunch at a nicer restaurant (at least a restaurant that doesn't frequent ask if you want fries with that?) is going to have some influence.