Congress has no obligation to do anything. Under the United States constitution, congress is the preeminent branch of government, clothed in the greatest and most significant powers, because it is closest to the people, with the house being elected from local districts every two years.
Congress is doing its job. They can make as little or as much new law as they like, and the voters will hold them accountable. If the voters want congress to alter the minimum wage, they must elect representatives who will vote to alter the minimum wage.