Why didnt people move out of slums?

Generally speaking, those who can afford to, do so as soon as they can.

Slums are generally places where there is great poverty.

The people who live in them cannot afford to move out. Property prices are too high for them elsewhere, and the property owners who own the slums don't have an incentive to improve their property as the rents are so low.
They didn't had enough money People did not move out of the slums because property prices were much higher elsewhere.