Why is a hamburger a greater risk for E.coli contamination than a steak?

Because the meat is ground, there is more surface area for bacteria to grow on.

Because cows have an immune system, for a healthy cow the meat can be assumed sterile until cut from the animal. Cutting exposes the "sterile" meat to bacteria, which can grow on the surface and slowly into the meat. Grinding exposes more surface area for bacterial growth.