The prime reason for failure has been the conflict between seeing the people who live in slums as victims and seeing them as perpetrators. Another was that slums were created by people without the skills and motivation to maintain decent living standards. Perhaps the least understood aspect is that it isn't just an urban problem.
Rural slums are largely invisible.
They are dispersed along little traveled roads and conditions are often worse than in cities. Now, of course, there are suburban slums too. Yet these slums are inextricably linked. When somebody eliminates a rural slum, perhaps simply leveling a few shacks, chances are the nearest city or suburban slums gain a few new residents. The same is true in reverse.