Following is not my observation. I read it somewhere (I wish I had remember). I think this is a very important observation about pivot role of cinema in modern societies.
After 2 wars with Pakistan India was passing through a very rough state of affairs in early and mid 1970s. Poverty was at peak, population was exploding, people were literally living on streets, alcoholism and prostitution was at rampage, basic necessities of life were counted as luxury and corruption was way of life. For poor men cinema was the only cheap entertainment but it was all glossy with romance. Amitabh came on screen as a rebellious young angry man who defies all norms of society and fight against injustice. His movies Zanjeer, Sholay, Deewar, Don etc were prime examples. Indian society instantly identified its hidden frustration on cinema screen. They found Amitabh as reflection of their fantasies. His long stature, deep voice and expressive angry face was in perfect coordination with inner desires of people.