Why does rauschenbusch argue that “the salvation of individuals” is not sufficient to address social problems?

Mr. Walter Rauschenbusch is known as the father of the social concern movement in America.

He served for eleven years as pastor of the second Baptist Church in New York City. Acknowledged as a loving pastor and social prophet, he did much to comfort the afflicted and afflict the comfortable.  

The ideal of the Kingdom of God, he said is not identified with  any  special social theory.

One author has commented about the question:

That the salvation  is a secure way to be with the Lord and it is guaranteed for all christian but here in the social living you have to confront many troubles day by day where the salvation is not very useful to resolve them.