I think that it is natural for Wall Street companies to actively seek Ivy League graduates. Ivy League schools have a reputation for recruiting and educating some of the best students. Quite naturally Wall Street would want to take advantage of the intellect that was produced by those institutions. By hiring the best candidates it ensures that the Wall Street companies keep up with the ever changing world.
Wall Street employers are more biased to go for an Ivy League graduate than someone who comes from any other type of school as they feel that Ivy League education is more quality than say your state school or community college. This has been established since the emergence of Wall Street in the early 20th century and probably will not change in the near future.
Yes, Wall Street firms are biased towards the Ivy League, because they are elitist and snobs. When really, most of going to an Ivy League school is knowing the right people and being able to pay for it. The Ivy League is an exclusive little club for the rich and powerful, and that's how Wall Street wants it to stay.
I do not think you can make a blanket statement saying Wall Street companies are biased towards Ivy League grads. It is my belief that all people are incharge of their own fate. If they make a big enough statement, I believe they will be hired. I believe that is true right down to the resume, if you make yourself stand out I feel it won't matter where you went to school.