Why are other races racist to Quarians?

Simply put, They are leechers.

Part of their pilgrimage is to venture out in search of useful technology or resources to being back to the flotilla. They contribute little to nothing to the rest of the galaxy. Plus you can add in the fact that they created the Geth which in the eyes of most people in the galaxy the Geth are a bunch of killing machines with only 1 purpose.

Make sure you look at it from a normal galactic civilian's point of view, and not Command Shepard's point of view.

I would think it's because of the flotilla. Since it has limited resources, they tend to mine any planets they come across, which doesn't please the residents. Maybe when C-sec see a quarian they think they're stealing for the flotilla!