In China, if illegal squatters get in the way of progress — by, say, building a home where the airport plans to expand — they disappear.
Democratic India doesn’t tackle the problem in quite the same way. From In India, the Path To Growth Hits Roadblock: Slums:
In Mumbai, the city formerly known as Bombay, the paupers have real political clout. Slum-dwellers constitute half of Mumbai’s 12 million citizens, and they are faithful voters. That makes them an important bloc for local politicians, most of whom promise to fight efforts to relocate them.
Another reason why there are so many slums:
Slums provide the drivers, maids and mechanics that keep Mumbai running. The tough part is housing them all. Rent control and strict building codes make low-cost housing a high-risk, low-return business.
The slums having running water, public toilets, electricity, and Hindu temples:
Peppering slums with temples is also a standard ploy to ensure that Hindu nationalists will rally to fight any threat of demolition.