Because it is based on a glaringly faulty behavioral model claiming that people must be prodded by hardship to act (for otherwise they grow complacent with the status quo) - which is the essence of the worse the living condition of the masses the better the chance for a rebellion school. This model exists largely in heads of armchair revolutionaries and has no shred of empirical support in social science. In fact, evidence point in the other direction - rebellion is most likely in situations when things are getting decidedly better but not quickly enough. The betterment of the human condition raises the expectations and leads people to believe everything is possible, the slow progress irritates them and prompts them to act. Another reason: any social movement for a change needs material resources and these are difficult to procure when things are going down - people are more likely to use them for their own protection rather than to advance a common good (that pesky free-rider problem). When things are getting better, however, more resources can be directed toward broader political goals.
Another reason is my total disgust with the cretinous sermons that there are no differences between political parties and candidates, and that we need to elect a new set of men (sometimes women) to change things for the better. This infantile babbling is totally oblivious of how things work in real life. It is a Left version of gossip tabloids where everything social and political is reduced to individual and personal.
Again, from a social science perspective (which I tried to briefly outline in my original posting) policy outcomes are not willed by individuals, but result from a complex interaction between competing interests, power, environmental constraints and rationalizations. Evidently, the primitive mind has difficulties comprehending that and reduces this complexity to the wills of individuals. That is why I called it religious thinking - it explains a complex reality it cannot understand with things that it can, namely human-like emotions of powerful human-like actors i.e. gods.
Wojtek