Why did members of Congress come to the defense of United Defense?

Members of Congress came to the defense of the Crusader project by United Defense both for the sake of jobs in home districts and because they saw the plans to cancel Crusader as a quick decision from the top (the Secretary of Defense) without proper consideration and analysis by other members of government.

Your question can be pursued further by viewing the "Interest Groups" segment of the Annenberg-Learner series, Democracy in America.

Crusader was an artillery system being developed by the US military, being built by a company called United Defense. Secretary of Defense Donald Rumsfeld wanted to go in a different, more streamlined and cost-efficient direction with the military. But Rumsfeld's plan to cancel the Crusader program ran into problems with the "iron triangle" of the army itself, the defense industry (represented here by the United Defense company), and members of Congress who challenged the cancellation of the project.

In the end, a compromise was worked out in which the full Crusader project was ended, but the contractor, United Defense, retained $475 million dollars to continue development of the Crusader's cannon. So the "iron triangle" lost this particular battle, in a sense, but maintained power in the ongoing "war" over how defense spending is decided.