Immigration as an issue stirs passions because people have to pick between American principles. Reform requires negotiating beliefs about liberty and security, law and morality, and opportunity and inclusiveness. Immigration reform impacts central duties of the federal government in both domestic and foreign policy.
Politicians crafting public policy have to construct coalitions to create a comprehensive bill. Negotiations over such a bill can breakdown in an instant. While the President and Congress can agree to certain goals, passing a bill with such a variety of interested parties calls for a legislative sleight of hand to prevent micro policy decisions from becoming macro political problems.