I disagree with that statement. Eastern European societies under the Soviet rule were remarkably free from hardship, yet a push toward social change there was remarkably strong. The above argument ignores the fact that 'social change' does not necessarily involve violent protest action - as many dinosaurs from the 1960s seem to think. It involves all kinds of changes; from family roles to input in the political process.
While it is true that Western democracies do not experience many violent protest actions, such as street fighting and protest, mass civil disobedience, guerilla warfare etc. - the main reason for that is that such high transaction cost means are not necessary, and in fact counterproductive, as there are other effective means of implementing change that do not require such high transaction costs - unless of course fighting and creating disruption becomes an end in itself.
I would go as far as arguing that Western democracies underwent more social change at all levels - from persons, to family, to doing business to politics than any other country in the world. Of course that does not square very well with the wet dreams of armchair revolutionaries who picture themselves on barricades surrounded by their followers. But that is cult-mentality, not social change.
Another factor is the intensity and effectiveness of propaganda. US is perhaps one of the most heavily propagandized society on the face of the Earth, and that propaganda diffuses and channels any attempt at social change. So the relative reluctance of many people to embrace social change here is more likely to be a result of propaganda rather than lack of hardship. In fact, people experiencing the most hardship are usually those who are most resistant to social change (cf. rural areas of PA and I am pretty sure other "backward" regions), while middle class is probably over-represented in various social movements. The same can be said about the Islamist movement - it recruits mainly middle class followers, not the downtrodden masses.
Wojtek