I would believe that Congress has continued to approve or extend the act because of the concern about threats of being called racist if they did not.  In our current climate when people who disagree with policy of our President are called racist, it is understandable that there would be this concern.  The media would supply lots of free accusations against any who voted to invalidate or not extend this voting act.  The reasoning supplied is validated by the voting registration and voting participation numbers of the black community over the past twenty years.